FileSystemAccessServer: Return EPERM for denied prompts

Instead of showing an ambiguous "Unknown error" when FSAS approval is
denied, let's affirm the user's action wasn't permitted if they
reject the prompt.

Userland/Services/FileSystemAccessServer/ConnectionFromClient.cpp

@@ -83,7 +83,7 @@ void ConnectionFromClient::request_file_handler(i32 request_id, i32 window_serve
             async_handle_prompt_end(request_id, 0, IPC::File(*file.release_value(), IPC::File::CloseAfterSending), path);
         }
     } else {
-        async_handle_prompt_end(request_id, -1, Optional<IPC::File> {}, path);
+        async_handle_prompt_end(request_id, EPERM, Optional<IPC::File> {}, path);
     }
 }