improve `upgrade` error message

[aryairani]

proj/branch> upgrade <old> <new>

  I couldn't automatically upgrade <old> to <new>.

proj/upgrade-old-to-new>

Should be something like.

proj/branch> upgrade <old> <new>

  I couldn't automatically upgrade <old> to <new>.

  I created a new branch for you to finish the migration; the new branch only has `<new>` and not `<old>`.
  Once you `update` your changes into the new branch, you can merge the result into your original branch
  with `merge <proj/upgrade-old-to-new> <proj/branch>`.

proj/upgrade-old-to-new>

[ceedubs]

#4644 is related in the sense that if it is implemented, the error message should reference it.