rust: error for WdirUnsupported with cpython conversion as exception
This introduces WorkingDirectoryUnsupported as an enum variant
of GraphError in the core and converts it to the expected
`mercurial.error.WdirUnsupported`.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state