message: Minor fixes to process_function_library_runtime
diff: @@ -350,10 +350,13 @@ Status ProcessFunctionLibraryRuntime::PinArgsAndRets(
  }
  std::vector<Device*> matching_devices;
  device_set.FindMatchingDevices(parsed, &matching_devices);
  - if (matching_devices.size() != 1) {
  + if (matching_devices.empty()) {
  + return errors::InvalidArgument(
  + "Unable to find any devices for spec ", *src_device);
  + } else if (matching_devices.size() != 1) {
  // Convert a vector of devices to a string.
  // Using absl::StrJoin did not work in Android builds.
  - string devices = "]";
  + string devices = "[";
  for (Device* device : matching_devices) {
  devices.append(device->name());
  devices.append(", ");
  