Private/nonexistent branches produce confusing errors

[Ryan Finnie]

When trying to run against a branch which is private and not accessible (or nonexistent), the following is returned:

2015-08-11 21:58:12 [ERROR] Unknown error
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/mojo/cli.py", line 668, in run_with_args
    args.func(args)
  File "/usr/lib/python2.7/dist-packages/mojo/cli.py", line 407, in run_from_manifest
    logfile=logfile)
  File "/usr/lib/python2.7/dist-packages/mojo/__init__.py", line 609, in create_workspace
    spec.initialize(clean_spec)
  File "/usr/lib/python2.7/dist-packages/mojo/__init__.py", line 82, in initialize
    self.local_branch.revert()
AttributeError: 'NoneType' object has no attribute 'revert'

This is what happens when I try to branch directly using bzrlib, which better hints at the problem:

bzrlib.errors.NotBranchError: Not a branch: "bzr+ssh://bazaar.launchpad.net/~a/b/c/".