proto: Add CherryPickError type for UserCherryPick structured errors
In UserCherryPick, there are some error conditions where instead of returning an error, we return OK and embed the error in the response. Instead of doing this, we want to return structured errors.
There a couple of places in the code where we are currently returning nil
as an error but we would like to return a structured error:
This change adds the proto definition for this error.
Edited by John Cai