Show the reason conflicts couldn't be resolved in the UI

We only support resolving conflicts in these cases: https://docs.gitlab.com/ce/user/project/merge_requests/resolve_conflicts.html#conflicts-available-for-resolution

• The file is text, not binary
• The file is in a UTF-8 compatible encoding
• The file does not already contain conflict markers
• The file, with conflict markers added, is not over 200 KB in size
• The file exists under the same path in both branches

While we can't show every reason that a conflict isn't resolvable in the UI, it is conceivable that we could show the first one. We could combine this with https://gitlab.com/gitlab-org/gitlab-ce/issues/22901.