cmd/release: check that a gomote token exists before uploading

It's helpful to check that required credentials exist before starting
the upload operation. That way, if the gomote token is missing, the
user will be told right away, rather than the operation failing midway.

For golang/go#45893.

Change-Id: I70aec0a9021a714032f2d0a9a8e51465cefd262e
Reviewed-on: https://go-review.googlesource.com/c/build/+/315929
Trust: Dmitri Shuralyov <[email protected]>
Trust: Alexander Rakoczy <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Reviewed-by: Alexander Rakoczy <[email protected]>

Author: dmitshur

cmd/release/release.go

@@ -61,6 +61,7 @@ func main() {
 
 	if *uploadMode {
 		buildenv.CheckUserCredentials()
+		userToken() // Call userToken for the side-effect of exiting if a gomote token doesn't exist.
 		if err := upload(flag.Args()); err != nil {
 			log.Fatal(err)
 		}