Fix Application Change Stage Error Responses

application.changeStage now returns an actionable error, instead of an internal server error, when the stage change cannot be completed. The application's stage is not changed in any of these cases:
- invalid_input when archiveEmail is provided for a candidate without a primary email address.
- invalid_input when moving an application to a Hired stage would bypass a required offer approval.
- invalid_input when moving an application to a Hired stage requires filling an opening.
- opening_in_use when moving an application to a Hired stage would fill an opening that is already in use by another application.