cmd/headscale/cli: move errMissingParameter and Error type to their users

Move errMissingParameter from users.go to utils.go alongside the
other shared sentinel errors; the variable is referenced by
api_key.go and preauthkeys.go.

Move the Error constant-error type from debug.go to mockoidc.go,
its only consumer.

cmd/headscale/cli/debug.go

@@ -9,11 +9,6 @@ import (
 	"github.com/spf13/cobra"
 )
 
-// Error is used to compare errors as per https://dave.cheney.net/2016/04/07/constant-errors
-type Error string
-
-func (e Error) Error() string { return string(e) }
-
 func init() {
 	rootCmd.AddCommand(debugCmd)