Merge request view when source branch non-existent or in error

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Description

• How should the merge request look when the source branch doesn't exist anymore, or there are errors.
• As in https://gitlab.com/gitlab-org/gitlab-ce/issues/30368 and https://gitlab.com/gitlab-org/gitlab-ce/issues/25575.
• Probably show description and discussion tab with comments.
• The commits tab should probably not be visible.
• Some type of error message in the merge request widget.
• Remove some irrelevant UI in the merge request widget.

Currently it looks like this: