"mojo run" fails in a non-obvious way if it doesn't understand the spec branch you're passing it

[Tom Haddon]

If you pass a non-spec branch location to "mojo run" it raises a non-obvious error:

Traceback (most recent call last):
  File "/usr/bin/mojo", line 9, in <module>
    load_entry_point('mojo==0.1', 'console_scripts', 'mojo')()
  File "/usr/lib/python2.7/dist-packages/mojo/cli.py", line 510, in main
    run_with_args(args, ap)
  File "/usr/lib/python2.7/dist-packages/mojo/cli.py", line 485, in run_with_args
    args.func(args)
  File "/usr/lib/python2.7/dist-packages/mojo/cli.py", line 261, in run_from_manifest
    args.name, stage=stage, manifest_file=args.manifest_file)
  File "/usr/lib/python2.7/dist-packages/mojo/__init__.py", line 465, in create_workspace
    mkdirs(ws_path, self.owner)
  File "/usr/lib/python2.7/dist-packages/mojo/utils.py", line 157, in mkdirs
    raise OSError("Creation of directory failed: {}".format(path))

It should instead tell you that you've passed it a non-spec branch location, or one it doesn't understand.

[David Ames (thedac)]

This bug has been around since early revisions. Brad Crittenden's revno 120 started on the fix but had the order of operations wrong.

Two problems existed. One the args.spec_url.rsplit("/")[-1] was always broken for no trailing slash. And two, if there was a trailing slash the guessed project name superseded the MOJO_PROJECT variable.

Set the priority for acquiring the project name
 1) From CLI with --project
 2) From MOJO_PROJECT environment variable
 3) Guess from the spec url and warn this is a guess

I am tempted to remove 3 altogether but it may be useful and now with better error handling it will be clear if that guess is incorrect.

lp:~thedac/mojo/non-spec-branch-location-fix