Fix MissingTemplate error in Gitaly error handler

What does this MR do and why?

Fixes a 500 ActionView::MissingTemplate error raised when Gitaly is unavailable during rapid diffs requests.

HandlesGitalyErrors#handle_gitaly_error responds to HTML requests with render action: action_name, which assumes every action has a template. Streaming and component-rendering actions have none. The rapid diffs diffs_stream and diff_file actions stream directly or render ViewComponents, so a Gitaly error there raised ActionView::MissingTemplate.

The fix checks template_exists? first and falls back to a plain-text 503 carrying the existing Gitaly-unavailable message.

How to set up and validate locally

1. Stop Gitaly:

   gdk stop gitaly

2. Request a compare diffs stream, for example http://gdk.test:3000/gitlab-org/gitlab-test/-/compare/diffs_stream?from=master&to=feature&straight=true

3. Before this change the response is a 500 (ActionView::MissingTemplate). After it, a 503 with the Gitaly-unavailable message.

4. Restart Gitaly:

   gdk start gitaly

Or run the specs:

bin/rspec spec/controllers/concerns/handles_gitaly_errors_spec.rb
bin/rspec spec/requests/projects/compare_diffs_stream_controller_spec.rb
bin/rspec spec/controllers/projects/compare_controller_spec.rb

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