Code review comment for lp:~mnn282/bzr/sftp-unsupported-operation-more-info

Martin Packman: So, the handling has been moved into _translate_io_exception. I also added tests, as you requested.

I was told on IRC that I should make _translate_io_exception static/class method, so tests actually won't be depended on SFTP transport itself - I had to make Transport._translate_error static too, because _translate_io_exception was invoking it.