buildlet: log something useful for quota errors

As implemented, it just logs a pointer address.

Updates golang/go#48857

Change-Id: Iba5bc8dbbb7a8eb7b5c110cd34233731e4f534ef
Reviewed-on: https://go-review.googlesource.com/c/build/+/428114
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Jenny Rakoczy <jenny@golang.org>

buildlet/gce.go

@@ -45,7 +45,7 @@ type GCEError struct {
 }
 
 func (q *GCEError) Error() string {
-	return fmt.Sprintf("failed with errors: %v", q.OpErrors)
+	return fmt.Sprintf("failed with errors: %+v", q.OpErrors)
 }
 
 func (q *GCEError) Is(target error) bool {