MDL-58899 auth: improve lang strings for confirmation failures in oauth

auth/oauth2/confirm-account.php

@@ -85,7 +85,7 @@ if ($confirmed == AUTH_CONFIRM_ALREADY) {
     echo $OUTPUT->footer();
     exit;
 } else {
-    print_error('invalidconfirmdata');
+    \core\notification::error(get_string('confirmationinvalid', 'auth_oauth2'));
 }
 
 redirect("$CFG->wwwroot/");

auth/oauth2/confirm-linkedlogin.php

@@ -72,7 +72,7 @@ if ($confirmed) {
     echo $OUTPUT->footer();
     exit;
 } else {
-    print_error('invalidconfirmdata');
+    \core\notification::error(get_string('confirmationinvalid', 'auth_oauth2'));
 }
 
 redirect("$CFG->wwwroot/");

auth/oauth2/lang/en/auth_oauth2.php

@@ -42,6 +42,7 @@ line at the top of your web browser window.
 If you need help, please contact the site administrator,
 {$a->admin}';
 $string['confirmaccountemailsubject'] = '{$a}: account confirmation';
+$string['confirmationinvalid'] = 'The confirmation link is either invalid, or has expired. Please start the login process again to generate a new confirmation email.';
 $string['confirmationpending'] = 'This account is pending email confirmation.';
 $string['confirmlinkedloginemail'] = 'Hi {$a->fullname},