Show conflicted files on pull request ui.

[lunny]

Currently when a PR is creating, it will return 500 if there are some conflicts. It's not very friendly UI.

[tboerger]

If it displays a 500 error page it's a bug and not a feature :)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs during the next 2 weeks. Thank you for your contributions.

[adelowo]

Un-stale