statushandler: Don't wrap context errors

When returning a structerr, we will check for wrapped errors and return the inner gRPC code when present. However, with context errors we want ensure that the Canceled/DeadlineExceeded statuses override any previous generated codes.

Wrapping the errors caused elevated error rates on InfoRefsUploadPack as the inner error of canceled processes was Internal, triggering incident #8566.

Use '%v' in our format strings for context-related errors so that we return the expected status.