Check for base_dir attr when opening menu in RabbitVCS

This is the cause of a lot of spurious Python exceptions when running
RabbitVCS. They are harmless, but should be checked since they fill up
.xsession-errors
Merge pull request #37 from wrouesnel/nemo-rabbitvcs-errormsg-fix

Check for base_dir attr when opening menu in RabbitVCS