Fix "Username already taken." validation error (could not happen)

The system.check_name() check *also* verifies that the username is not
already taken, so we must perform the matchuser() check first in order
to get the appropriate error message here when trying to use an alias
(username) that's already taken.

webv4/root/api/register.ssjs

@@ -90,10 +90,10 @@ if (system.newuser_password !== '' && (!request.has_param('newuser-password') ||
 	reply.errors.push(locale.strings.api_register.error_bad_syspass);
 }
 
-if (!valid_param('alias', MIN_ALIAS, LEN_ALIAS) || !system.check_name(clean_param('alias'))) {
-	reply.errors.push(locale.strings.api_register.error_invalid_alias);
-} else if (system.matchuser(clean_param('alias')) > 0) {
+ if (system.matchuser(clean_param('alias')) > 0) {
 	reply.errors.push(locale.strings.api_register.error_alias_taken);
+} else  if (!valid_param('alias', MIN_ALIAS, LEN_ALIAS) || !system.check_name(clean_param('alias'))) {
+	reply.errors.push(locale.strings.api_register.error_invalid_alias);
 } else {
 	prepUser.alias = clean_param('alias');
 	prepUser.handle = clean_param('alias');