improve excpeiton message

Author: TomasVotruba

packages/PostRector/Application/PostFileProcessor.php

@@ -64,7 +64,12 @@ private function sortByPriority(array $postRectors): array
 
         foreach ($postRectors as $postRector) {
             if (isset($postRectorsByPriority[$postRector->getPriority()])) {
-                throw new ShouldNotHappenException();
+                $errorMessage = sprintf(
+                    'There are multiple post rectors with the same priority: %d. Use different one for your new PostRector',
+                    $postRector->getPriority()
+                );
+
+                throw new ShouldNotHappenException($errorMessage);
             }
 
             $postRectorsByPriority[$postRector->getPriority()] = $postRector;