Internationalize Commit API error messages

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Problem to solve

As was raised in this discussion the commit API surfaces Gitaly error messages to the user which are not translated.

Proposed solution

Internationalize the commit API error messages received from Gitaly.

Implementation plan

• Update the index_error_message method in lib/gitlab/gitaly_client/operation_service.rb to return translated messages.
• Update use cases of the messages in the frontend.