Fix unmarshaling and displaying of gitserver CreateCommitFromPatchError

Created by: mrnugget

Before this change I ran into this error message:

frontend | WARN decoding gitserver create-commit-from-patch response, err: json: cannot unmarshal object into Go struct field CreateCommitFromPatchError.Error.Err of type error

This commit changes the field from an error to a string, which can be marshaled/unmarshaled by the json package.

I don't see where we need to satisfy the Unwrap() interface and preserve the original error, so in order to make this work, I got rid of it.