Avoid AttributeError with py3
AttributeError: 'ValueError' object has no attribute 'message' ocurrs during an exception in extra_qwt.image classes. Message member does not exist in Py3.
Fix it, using string representation instead.
AttributeError: 'ValueError' object has no attribute 'message' ocurrs during an exception in extra_qwt.image classes. Message member does not exist in Py3.
Fix it, using string representation instead.