message: Surround device type in quotes
  This makes it more obvious when the device type is (incorrectly) an empty string as it happened in a recent bug report.
diff: @@ -1477,8 +1477,8 @@ Status CreateOpKernel(DeviceType device_type, DeviceBase* device,
  }
  if (registration == nullptr) {
  s.Update(errors::NotFound("No registered '", node_def.op(),
  - "' OpKernel for ", DeviceTypeString(device_type),
  - " devices compatible with node ",
  + "' OpKernel for '", DeviceTypeString(device_type),
  + "' devices compatible with node ",
  FormatNodeDefForError(node_def)));
  if (was_attr_mismatch) {
  errors::AppendToMessage(
  