Unwrap the InvocationTargetException improving the error message

[lukecwik]

Unwrap the InvocationTargetException improving the error message which users get when their application fails.

This removes the boilerplate found in the plugin output which preceeds the users stacktrace:

java.lang.reflect.InvocationTargetException
        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:498)
        at org.codehaus.mojo.exec.ExecJavaMojo$1.run(ExecJavaMojo.java:293)
        at java.lang.Thread.run(Thread.java:745)
... <Users Stack Trace> ...

making it just contain the stack trace from the users application.

This also removes null: InvocationTargetException: from the plugin output summary:

An exception occured while executing the Java class. null: InvocationTargetException: <Message from users exception>

thus making it:

An exception occured while executing the Java class. <Message from users exception>