Follow-up from "Use Gitaly Exception Message to Check for Path"

The following discussion from !128366 (merged) should be addressed:

• @ashmckenzie started a discussion:

  suggestion (nitpick): Calling stub_feature_flags(handle_structured_gitaly_errors: <value>) multiple times could be extracted into a single call in a before block under the when path does not exist context. Each context would then call let(:handle_structured_gitaly_errors) { <true|false> }, e.g.

          let(:handle_structured_gitaly_errors) { nil }
  
          before do
            stub_feature_flags(handle_structured_gitaly_errors: handle_structured_gitaly_errors)
          end
  
          context 'when handle_structured_gitaly_errors feature is disabled' do

  Let's handle this as a follow-up