Make all errors over gob consistent in gitaly-git2go

Part of #3229 (closed)

Eventually we are looking to output logs over stderr. Right now stderr is used for plain text errors when combined with an exit code. Here we are making sure all gob result types have a compatible error field. Once this has been released we will then be able to stop using stderr for errors and encode these errors over gob instead.

We couldn't continue to use the field name Error as this was already in use. So this MR introduces a new field that can be consistent.