The star labels 'star' (default star) and 'ignore' (for ignoring a
change) are mutually exclusive. Trying to set one if the other is
already set failed with an internal server error. Handle this case and
return 409 Conflict instead.
In addition this adds a test to verify that setting an invalid star
label is correctly rejected with 400 Bad Request. This case was already
correctly handled in the Stars.Post REST endpoint (sets multiple stars
at once), but not in the StarredChanges.Create REST endpoint (adds a
single star).
Bug: Issue 7238
Change-Id: Id4bb5604116f4959d8ecba66146bab72437205f6
Signed-off-by: Edwin Kempin <ekempin@google.com>
c8cfdb0855