vcs: Deprecate generic missing commit merge error reason. There are two more specific error reasons (target/source ref missing) which should be used instead.

vcs: Use more specific merge error reason (either target or source ref missing)

tests: Add test to check PR page when target reference is missing. The pr page should display the desired error message to the user.

tests: Fix test by using flash message text from model.

tests: Adapt tests to return value change.

pr: Use new update response object from pr model. #3950

pr: Catch errors if target or source reference are missing during commit update. #3950

pr: Return update response objects instead of tuples.

pr: Add UpdateResponse data structure to hold response data when updating commits.

pr: Add update status messages dict.