Hub could inform user of `upstream` convention in errors

[seadowg]

For features like pr checkout Hub relies on you having an upstream remote setup. This is often the case but if you set up a repo locally using hub you might end up (as I did) with your "upstream" named after the org due to a hub fetch <org-name>. Of course then when you got to do a hub pr checkout you'll get hit with a 404.

This is definitely a mistake on the user's part but Hub could help with an additional line in the resulting error message with something like "is your upstream remote pointed at the correct repository?".

[mislav]

Thank you for the feedback! I agree it's currently a mess how hub handles remotes, the detection of upstream repo, and related context. At least when it fails, it could give people some pointers about what went wrong. 👍