Add an explicit exception/warning when the uploader produces empty output

[newgene]

As described in #357, when the uploader parser failed to process the data file(s), there could be a case that the parser does not raise any parsing error but does not produce any valid output (e.g. could be the invalid data files during the dump stage). In this case, a profound error of No temp collection (or it's empty) will be raised.

I think we should capture this error early, maybe in post_upload, and produce an explicit error message. This is particularly useful when a user is testing a data plugin using our CLI.