In PR, the error message is not correctly mentioned

When a PR is sent from a fork to the original repo and on the original repo the "links" is required, when I try to merge the PR on the original repo using "Fast-forward" merge-strategy, I get an error in Chrome's Console output, instead of telling me that it's requiring an issue key

How to Replicate

1. Create a repo R1 with content (for example with a README file) with user U1 under team T
2. Enforce requiring issue keys in commit messages (underneath Settings / Links) on R1
3. Invite user U2 to team T
4. User U2 forks repo R1 to repo R2
5. User U2 pushes a commit to his/her own fork (R2) without an issue key
6. User U2 send a PR back to the main repo (R1)
7. User U1 tries to merge PR with Fast-forward from U2/R2
8. It's going to fail, but can be merged with "Merge commit" strategy

Issues:

1. I'm not supposed to merge it with "Merge commit" strategy as it should require an issue link
2. I'm not supposed to be able to send this PR from fork to the original repo as it doesn't have issue link
3. When we try to merge it with "Fast-Forward" strategy, instead of mentioning "Something went wrong", we should mention "Issue key is required"