blob: Standardize returned errors

Replace different representation of the missing repository argument error with the pre-defined ErrEmptyRepository. Fix order of arguments validation with localrepo creation in GetBlob. Replace status.Error(f) with helper.ErrXXX to have a common pattern and omit possible double-wrapping of the gRPC Status. RPC name removed from the error and add a more valuable prefix is added with description of the failed operation.

The change of the return error message doesn't affect GitLab code, at least the parts covered with tests.
The GitLab's pipeline works fine with this change.
The QA also completes without issues caused by the change.