[5.6] change error message for auth driver not defined (#23455)

* change error message wording to make it clear that the driver is not defined

* include the name of the undefined driver in error

src/Illuminate/Auth/AuthManager.php

@@ -94,7 +94,7 @@ protected function resolve($name)
             return $this->{$driverMethod}($name, $config);
         }
 
-        throw new InvalidArgumentException("Auth guard driver [{$name}] is not defined.");
+        throw new InvalidArgumentException("Auth driver [{$config['driver']}] for guard [{$name}] is not defined.");
     }
 
     /**