JavaHL: Exceptions in LogMessageCallback.singleMessage should abort the log immediately

[Marc Strapetz]

If e.g. a RuntimeException is thrown in 
LogMessageCallback#singleMessage, it's not processed in 
LogMessageCallback::singleMessage and the log is continued nevertheless:

(1) At line 77 in LogMessageCallback.cpp, there should be returned an 
appropriate error code.

(2) After line 122, JNIUtil::isJavaExceptionThrown() should be called 
and there should be returned an appropriate error code.

In both cases, the returned error code should result in stopping the 
low-level log; rethrowing the Exception in RemoteSession::getLog won't 
be necessary, as this can be established easily from within client code 
itself.

-Marc