cloud agent: Handle 422 errors gracefully.

[joshspicer]

Does this issue occur when all extensions are disabled?: Yes/No

While we shouldn't get into the case (it's handled here) this is cheap insurance for an otherwise confusing error.

[joshspicer]

1. Create a branch in a workspace but DO NOT push it
2. trigger cloud agent
3. You will see a helpful error message stating that your branch is not pushed