Notify only merge request unmergeable due to conflict

Note: A 11.0 port MR is created to resolve conflict. It will be in 11.0.2

What does this MR do?

Only generate notification when MR becomes unmergeable due to conflict.

Are there points in the code the reviewer needs to double check?

1. Traditionally, if there is non-utf8 branch exception, it is raised during checking. However features/merge_request/user_resolves_conflicts_spec.rb:170 requires this to be ignored. So during checking, this type of error is rescued.
2. Inside repository can_be_merged? check, logic is added to check if target branch is present. See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19548#note_82558796 for more details.

Why was this MR needed?

It is discovered that "no-commit" and "source branch removed" situations are common and should not be considered as error, and users should not be notified of those situations. In the end I feel only merge request conflict really needs notification because it is the main use case.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Part of #47488 (closed)