message: Tweak error message on failure to export from MLIR to XLA: the language used "lowering" which usually refers to TF->HLO.
  Also use emitOpError which provides more information.
diff: @@ -1027,8 +1027,7 @@ LogicalResult ConvertToHloModule::Lower(
  return success();
  }
  - inst->emitError("unable to lower operation of type '" +
  - inst->getName().getStringRef().str() + '\'');
  + inst->emitOpError() << "can't be translated to XLA HLO";
  return failure();
  }
  