[JENKINS-16836] UpdateCenter REST API crashes when plugin installation failed

https://issues.jenkins-ci.org/browse/JENKINS-16836

When plugin installation failed for some reason, http://localhost:8080/updateCenter/api/json?tree=jobs[error] returns 500 Error.
Since UpdateCenter#UpdateCenterJob#getError returns Throwable instance, we should expose Throwable#getErrorMessage instead of itself.

This happens since https://github.com/jenkinsci/jenkins/commit/76832379d8c3ed019b7ef8f36935df31ea23d261 is released.

```
Status Code: 500
Exception: org.kohsuke.stapler.export.NotExportableException: class java.net.UnknownHostException doesn't have @ExportedBean
Stacktrace:

javax.servlet.ServletException: org.kohsuke.stapler.export.NotExportableException: class java.net.UnknownHostException doesn't have @ExportedBean
	at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:615)
	at org.kohsuke.stapler.Stapler.invoke(Stapler.java:658)
	at org.kohsuke.stapler.MetaClass$4.doDispatch(MetaClass.java:203)
	at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
	at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:573)
	at org.kohsuke.stapler.Stapler.invoke(Stapler.java:658)
	at org.kohsuke.stapler.MetaClass$4.doDispatch(MetaClass.java:203)
	at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
	at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:573)
	at org.kohsuke.stapler.Stapler.invoke(Stapler.java:658)
	at org.kohsuke.stapler.Stapler.invoke(Stapler.java:487)
	at org.kohsuke.stapler.Stapler.service(Stapler.java:164)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
	at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:598)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1367)
	at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:95)
	at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:101)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1338)
	at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1338)
	at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
	at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:90)
	at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1338)
	at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:50)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1338)
	at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1338)
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:484)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:119)
	at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:499)
	at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:231)
	at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1065)
	at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:413)
	at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:192)
	at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:999)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:117)
	at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:250)
	at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:149)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:111)
	at org.eclipse.jetty.server.Server.handle(Server.java:350)
	at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:454)
	at org.eclipse.jetty.server.AbstractHttpConnection.headerComplete(AbstractHttpConnection.java:890)
	at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.headerComplete(AbstractHttpConnection.java:944)
	at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:630)
	at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:230)
	at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:77)
	at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:620)
	at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:46)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:603)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:538)
	at java.lang.Thread.run(Thread.java:680)
Caused by: org.kohsuke.stapler.export.NotExportableException: class java.net.UnknownHostException doesn't have @ExportedBean
	at org.kohsuke.stapler.export.Model.<init>(Model.java:71)
	at org.kohsuke.stapler.export.ModelBuilder.get(ModelBuilder.java:48)
	at org.kohsuke.stapler.export.Property.writeValue(Property.java:225)
	at org.kohsuke.stapler.export.Property.writeValue(Property.java:137)
	at org.kohsuke.stapler.export.Property.writeTo(Property.java:114)
	at org.kohsuke.stapler.export.Model.writeNestedObjectTo(Model.java:187)
	at org.kohsuke.stapler.export.Model.writeNestedObjectTo(Model.java:182)
	at org.kohsuke.stapler.export.Property.writeValue(Property.java:232)
	at org.kohsuke.stapler.export.Property.writeValue(Property.java:182)
	at org.kohsuke.stapler.export.Property.writeValue(Property.java:137)
	at org.kohsuke.stapler.export.Property.writeTo(Property.java:114)
	at org.kohsuke.stapler.export.Model.writeNestedObjectTo(Model.java:187)
	at org.kohsuke.stapler.export.Model.writeTo(Model.java:154)
	at org.kohsuke.stapler.ResponseImpl.serveExposedBean(ResponseImpl.java:222)
	at hudson.model.Api.doJson(Api.java:192)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:288)
	at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:151)
	at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:90)
	at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:111)
	at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
	at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:573)
	... 51 more
```