Use getRootCause to un-obscure root cause exception.

[neofreko]

When automation test fail, graphwalker will always show: java.lang.reflect.InvocationTargetException. This makes debugging difficult as we have to rely on log message to see where the code fail -- often with guessing.

This small PR will attach the root cause instead of graphwalker local exception.

Given the following implementation:

Before:

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.graphwalker.core.machine.ExecutionContext.execute(ExecutionContext.java:277)

After:

java.lang.RuntimeException
	at org.graphwalker.java.test.TestExecutorTest$ThrowExceptionTest.throwException(TestExecutorTest.java:188)
	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.graphwalker.core.machine.ExecutionContext.execute(ExecutionContext.java:277)

[KristianKarl]

Thank you @neofreko 👍