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
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* 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
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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.
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