Fix unicode problems when processing errors from SVN.

When processing any errors that come out of the pysvn client, we would cast the
exception to unicode without specifying an encoding. This resulted in any utf-8
filenames or other text triggering a UnicodeDecodeError. I've changed this to
first coerce the exception to bytes, and then decode. While I was in here, I
also fixed up a duplicate cast.

Testing done:
Ran unit tests (this error handler is exercised in the tests).

Fixes bug 3518.

Reviewed at https://reviews.reviewboard.org/r/6196/