[feature/oauth] Start linking/registering OAuth accounts during login

PHPBB3-11673

phpBB/phpbb/auth/provider/oauth/oauth.php

@@ -172,7 +172,15 @@ class phpbb_auth_provider_oauth extends phpbb_auth_provider_base
 			if (!$row)
 			{
 				// The user does not yet exist, ask if they wish to register the account
-				throw new Exception($unique_id);
+				return array(
+					'status'		=> LOGIN_SUCCESS_LINK_PROFILE,
+					'error_msg'		=> 'LOGIN_OAUTH_ACCOUNT_NOT_LINKED',
+					'user_row'		=> array(),
+					'redirect_data'	=> array(
+						'auth_provider'	=> 'oauth',
+						'oauth_service'	=> $service_name_original,
+					),
+				);
 			}
 
 			// Retrieve the user's account