cat: Return a non-null value if an error occurred

Author: https://github.com/LucasChollet
Commit: https://github.com/SerenityOS/serenity/commit/1944e8936d
Pull-request: https://github.com/SerenityOS/serenity/pull/16169
Reviewed-by: https://github.com/trflynn89

Userland/Utilities/cat.cpp

@@ -45,5 +45,5 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         }
     }
 
-    return 0;
+    return files.size() != paths.size();
 }