PLT-488 - TestHelper assertThrows should output actual exception

When an exception is thrown but is not of expected type, the actual exception thrown is included in the failure message. Compare before:

```
java.lang.AssertionError: Expected SQLIntegrityConstraintViolationException but received BadSqlGrammarException
	at org.testng.Assert.fail(Assert.java:94)
	at com.opengamma.collect.TestHelper.assertThrowsImpl(TestHelper.java:225)
	at com.opengamma.collect.TestHelper.assertThrows(TestHelper.java:181)
	at com.opengamma.platform.masterdb.SchemaTest.documentVersionInsertion(SchemaTest.java:112)
```
with after:

```
java.lang.AssertionError: Expected SQLIntegrityConstraintViolationException but received BadSqlGrammarException
	at org.testng.Assert.fail(Assert.java:83)
	at com.opengamma.collect.TestHelper.assertThrowsImpl(TestHelper.java:225)
	at com.opengamma.collect.TestHelper.assertThrows(TestHelper.java:181)
	at com.opengamma.platform.masterdb.SchemaTest.documentVersionInsertion(SchemaTest.java:112)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:483)
	at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:84)
	at org.testng.internal.Invoker.invokeMethod(Invoker.java:714)
	at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:901)
	at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1231)
	at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:127)
	at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:111)
	at org.testng.TestRunner.privateRun(TestRunner.java:767)
	at org.testng.TestRunner.run(TestRunner.java:617)
	at org.testng.SuiteRunner.runTest(SuiteRunner.java:348)
	at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:343)
	at org.testng.SuiteRunner.privateRun(SuiteRunner.java:305)
	at org.testng.SuiteRunner.run(SuiteRunner.java:254)
	at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:52)
	at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:86)
	at org.testng.TestNG.runSuitesSequentially(TestNG.java:1224)
	at org.testng.TestNG.runSuitesLocally(TestNG.java:1149)
	at org.testng.TestNG.run(TestNG.java:1057)
	at org.testng.remote.RemoteTestNG.run(RemoteTestNG.java:111)
	at org.testng.remote.RemoteTestNG.initAndRun(RemoteTestNG.java:204)
	at org.testng.remote.RemoteTestNG.main(RemoteTestNG.java:175)
	at org.testng.RemoteTestNGStarter.main(RemoteTestNGStarter.java:125)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:483)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)
Caused by: org.springframework.jdbc.BadSqlGrammarException: PreparedStatementCallback; bad SQL grammar [insert into document_version (id, oid, version_from, corrected_from, bean_data) values (?, ?, ?, ?, ?)]; nested exception is java.sql.SQLSyntaxErrorException: incompatible data type in conversion
	at org.springframework.jdbc.support.SQLExceptionSubclassTranslator.doTranslate(SQLExceptionSubclassTranslator.java:95)
	at org.springframework.jdbc.support.AbstractFallbackSQLExceptionTranslator.translate(AbstractFallbackSQLExceptionTranslator.java:73)
	at org.springframework.jdbc.support.AbstractFallbackSQLExceptionTranslator.translate(AbstractFallbackSQLExceptionTranslator.java:81)
	at org.springframework.jdbc.core.JdbcTemplate.execute(JdbcTemplate.java:660)
	at org.springframework.jdbc.core.JdbcTemplate.update(JdbcTemplate.java:909)
	at org.springframework.jdbc.core.JdbcTemplate.update(JdbcTemplate.java:970)
	at org.springframework.jdbc.core.JdbcTemplate.update(JdbcTemplate.java:980)
	at com.opengamma.platform.masterdb.SchemaTest.lambda$documentVersionInsertion$1(SchemaTest.java:113)
	at com.opengamma.platform.masterdb.SchemaTest$$Lambda$1/1175371136.run(Unknown Source)
	at com.opengamma.collect.TestHelper.assertThrowsImpl(TestHelper.java:212)
	... 32 more
```