CDAP-608 Better error message when flowlet has no input

New error message:

```
2014-11-24 15:34:28,722 - ERROR [executor-19:c.c.c.g.h.AppFabricHttpHandler@881] - Flowlet saver in flow WhoFlow of application HelloWorld has no input
java.lang.IllegalArgumentException: Flowlet saver in flow WhoFlow of application HelloWorld has no input
	at co.cask.cdap.internal.app.runtime.flow.FlowletProgramRunner.createProcessSpecification(FlowletProgramRunner.java:366) ~[classes/:na]
	at co.cask.cdap.internal.app.runtime.flow.FlowletProgramRunner.run(FlowletProgramRunner.java:240) ~[classes/:na]
	at co.cask.cdap.internal.app.runtime.flow.FlowProgramRunner.startFlowlet(FlowProgramRunner.java:142) ~[classes/:na]
	at co.cask.cdap.internal.app.runtime.flow.FlowProgramRunner.createFlowlets(FlowProgramRunner.java:119) ~[classes/:na]
	at co.cask.cdap.internal.app.runtime.flow.FlowProgramRunner.run(FlowProgramRunner.java:97) ~[classes/:na]
	at co.cask.cdap.app.runtime.AbstractProgramRuntimeService.run(AbstractProgramRuntimeService.java:55) ~[classes/:na]
	at co.cask.cdap.internal.app.runtime.service.InMemoryProgramRuntimeService.run(InMemoryProgramRuntimeService.java:79) ~[classes/:na]
	at co.cask.cdap.gateway.handlers.AppFabricHttpHandler.start(AppFabricHttpHandler.java:852) [classes/:na]
	at co.cask.cdap.gateway.handlers.AppFabricHttpHandler.runnableStartStop(AppFabricHttpHandler.java:802) [classes/:na]
	at co.cask.cdap.gateway.handlers.AppFabricHttpHandler.startStopProgram(AppFabricHttpHandler.java:790) [classes/:na]
	at co.cask.cdap.gateway.handlers.AppFabricHttpHandler.startProgram(AppFabricHttpHandler.java:629) [classes/:na]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.7.0_51]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) ~[na:1.7.0_51]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.7.0_51]
	at java.lang.reflect.Method.invoke(Method.java:606) ~[na:1.7.0_51]
	at co.cask.http.HttpMethodInfo.invoke(HttpMethodInfo.java:83) [netty-http-0.6.0.jar:na]
	at co.cask.http.HttpDispatcher.messageReceived(HttpDispatcher.java:41) [netty-http-0.6.0.jar:na]
	at org.jboss.netty.channel.SimpleChannelUpstreamHandler.handleUpstream(SimpleChannelUpstreamHandler.java:70) [netty-3.6.6.Final.jar:na]
	at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:564) [netty-3.6.6.Final.jar:na]
	at org.jboss.netty.channel.DefaultChannelPipeline$DefaultChannelHandlerContext.sendUpstream(DefaultChannelPipeline.java:791) [netty-3.6.6.Final.jar:na]
	at org.jboss.netty.handler.execution.ChannelUpstreamEventRunnable.doRun(ChannelUpstreamEventRunnable.java:43) [netty-3.6.6.Final.jar:na]
	at org.jboss.netty.handler.execution.ChannelEventRunnable.run(ChannelEventRunnable.java:67) [netty-3.6.6.Final.jar:na]
	at org.jboss.netty.handler.execution.OrderedMemoryAwareThreadPoolExecutor$ChildExecutor.run(OrderedMemoryAwareThreadPoolExecutor.java:314) [netty-3.6.6.Final.jar:na]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [na:1.7.0_51]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [na:1.7.0_51]
	at java.lang.Thread.run(Thread.java:744) [na:1.7.0_51]
Caused by: co.cask.cdap.internal.app.runtime.flow.FlowletMissingInputException: Flowlet saver in flow WhoFlow of application HelloWorld has no input
	at co.cask.cdap.internal.app.runtime.flow.FlowletProgramRunner$4.create(FlowletProgramRunner.java:553) ~[classes/:na]
	at co.cask.cdap.internal.app.runtime.flow.FlowletProgramRunner.createProcessSpecification(FlowletProgramRunner.java:363) ~[classes/:na]
	... 25 common frames omitted
```
When need a new exception for that?