Distinguish between Gitaly being unavailable and other errors
Display different errors depending on the error response or status code.
It is not possible for the BE to provide a specific error message in the response when it's a Gitlay failure, so from the FE display the Gitaly error message when the project
, repository
and paginatedTree
paths are present in the error response.
Follow-up issue from #340816 (comment 700472707)