fix: clarify LRO parsing error messages

Author: miraleung

src/main/java/com/google/api/generator/gapic/protoparser/Parser.java

@@ -367,10 +367,16 @@ static LongrunningOperation parseLro(
  Message responseMessage = messageTypes.get(responseTypeName);
  Message metadataMessage = messageTypes.get(metadataTypeName);
  Preconditions.checkNotNull(
- responseMessage, String.format("LRO response message %s not found", responseTypeName));
+ responseMessage,
+ String.format(
+ "LRO response message %s not found on method %s",
+ responseTypeName, methodDescriptor.getName()));
  // TODO(miraleung): Check that the packages are equal if those strings are not empty.
  Preconditions.checkNotNull(
- metadataMessage, String.format("LRO metadata message %s not found", metadataTypeName));
+ metadataMessage,
+ String.format(
+ "LRO metadata message %s not found in method %s",
+ metadataTypeName, methodDescriptor.getName()));
 
  return LongrunningOperation.withTypes(responseMessage.type(), metadataMessage.type());
  }