When all samples fail lowread_filter, pipeline errors

[zxl124]

When all samples have less than params.lowread_filter reads, the pipeline errors at qiime_datamerge step. The ideal behavior should be the pipeline still finishes and produces report, but with a message on the comment section of the report, similar to how bad samples are handled right row.

[zxl124]

In a related note, when samples produce sourmash results, but the results are completely filtered out by strict filtering criteria, the sourmash_qiimeprep process produces empty files for qiime to import.