MWException: Cleanup exception message output

MWException: Cleanup exception message output

Change I0a9e9244 lost the message when handling a non-MWException
exception, and for a long time MWException's getHTML and getText have
been missing the actual file and line where the exception was thrown.

We may as well use MWExceptionHandler::getLogMessage to fix all of
these, resulting in a more standardized exception output.

Change-Id: I8a9b4831c9c586bafe0a54516ff779cdfb008984