If the_oserror.filename is not None then str(the_oserror) appends the
filename twice:

  [Errno 2] No such file or directory: 'nonexistent': 'nonexistent'

You could remove `err_msg += ':' ...` statements to avoid the
repeatition. It may break the code that uses strerror attribute. But
exception error messages are explicitly excluded from backward
compatibility considirations therefore it might be ok to break it
here. I can't find the reference so it should probably be resolved as a
new issue (independent from providing the filename attribute value).