Fix another SVN unicode problem.
Review Request #6326 — Created Sept. 15, 2014 and submitted
When there's a file-not-found error with unicode paths in SVN, we were trying
to format the encoded string into a unicode string to create the error message.
This caused a UnicodeDecodeError when it tried to coerce it using the 'ascii'
codec. I've changed theFileNotFoundError.__init__method to cope with paths
which are bytes as well as paths which are unicode.
Ran unit tests.
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