branch submit: better error message if base hasn't been submitted

[abhinav]

When base branch is non-trunk, but hasn't been submitted, GitHub will reject the submission in gs branch submit because it doesn't know the base ref and won't see any commits between base and head.

The error takes the form:

Head sha can't be blank, Base sha can't be blank, No commits between [...] and [...], Base ref must be a branch

gs branch submit should confirm that non-trunk base has been submitted before it submits the current branch, while still allowing for --no-publish for non-GitHub workflows.

[abhinav]

This error also occurs when base has been submitted and merged (and deleted server-side), but local hasn't synced yet.

So it can't be a naive approach like #499 where we just check if the base branch is submitted.
We'll need to look at the GitHub error code.