tool/gocross: fail if the toolchain revision isn't findable

This used to make sense, but after a refactor somewhere along the line
this results in trying to download from a malformed URL and generally
confusing failures.

Signed-off-by: David Anderson <danderson@tailscale.com>

tool/gocross/toolchain.go

@@ -39,9 +39,6 @@ findTopLevel:
 func readRevFile(path string) (string, error) {
 	bs, err := os.ReadFile(path)
 	if err != nil {
-		if os.IsNotExist(err) {
-			return "", nil
-		}
 		return "", err
 	}
 	return string(bytes.TrimSpace(bs)), nil