Fix a bug in JavaHL error handling; the original error was not cleared.
Enhance the native exception to SVN error wrapper to include the name
of the exception class in the error message.

* subversion/bindings/javahl/native/JNIUtil.h
  (JNIUtil::wrappedHandleSVNError): New private method.
* subversion/bindings/javahl/native/JNIUtil.cpp
  (JNIUtil::wrappedHandleSVNError): Renamed from JNIUtil::handleSVNError.
  (JNIUtil::handleSVNError): New implementation that clears the error.
  (known_exception_to_cstring): New; extracted from exception_to_cstring.
   Finds the exception class name.
  (exception_to_cstring): Call known_exception_to_cstring.
  (JNIUtil::checkJavaException): Call known_exception_to_cstring.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1532117 13f79535-47bb-0310-9956-ffa450edef68