Merge Request fails with cryptic error if author restriction is enabled

If a project is configured to use the following githook (via https://gitlab.com/group/project/git_hooks):

Check whether author is a GitLab user
Restrict commits by author (email) to existing GitLab users

Attempting to use the gitlab web interface to merge a branch with an offending commit fails, providing only the following information:
Something went wrong during merge

A more useful error message would inform the user this constraint has been violated. Gitlab access via git CLI already does so. Case in point:

remote: GitLab: Author 'email@example.com' is not a member of team
To git@gitlab.com:group/project.git
! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'git@gitlab.com:group/project.git'