Better error compliance and comparison tests refactor

This improves HGitaly compliance with Gitaly's behaviour

• when repository or storage are not to be found, including in the special case (not prone to really happen unless there is a bug in our client code) where a repository argument is missing. This question was raised in the process of implementing RepositoryExists in RHGitaly (can still be seen in !140 (merged)), which did not enjoy a Gitaly Comparison test.
• in the GetTreeEntry gRPC method, because our Gitaly Comparison test was buggy.

The Gitaly Comparison test module for blobs and trees is now using the comparison.RpcHelper class, a big refactoring effort that led to the introduction of the new assert_compare_aggregated helper method.