Clarify exception message

src/Composer/Repository/VcsRepository.php

@@ -228,7 +228,7 @@ class VcsRepository extends ArrayRepository
         $this->io->overwrite('', false);
 
         if (!$this->getPackages()) {
-            throw new \RuntimeException('No composer.json was found in any branch or tag of '.$this->url.', could not load a package from it.');
+            throw new \RuntimeException('No valid composer.json was found in any branch or tag of '.$this->url.', could not load a package from it.');
         }
     }