Handle GRPC::NotFound in Gitaly tree_entry call

What does this MR do?

Prepares the rails backend for gitaly!3078 (merged) so we can make that breaking change as part of %14.0.

Screenshots (strongly suggested)

Test setup: For all four combinations of this MR and gitaly!3078 (merged), visit a non-existing file

Results: As expected, 404 in all situations except when the gitaly change is applied without this MR, since that error is currently unhandled.

	without rails change	with rails change
without gitaly change
with gitaly change

Does this MR meet the acceptance criteria?

Conformity

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  • I have not included a changelog entry because this change is entirely invisible to the user.
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• Tested in all supported browsers
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