Add QC check if no reads are left after cutadapt

[slsevilla]

Problem

Recently had a project that had no reads left after trimming. The sample did not fail and continued downstream making it difficult to pinpoint where the sample had truly failed

Solution

After cutadapt add a check to ensure there are suitable number of reads to move on in the pipeline.

[kelly-sovacool]

this can happen if a file contains only adapters -- make a unit test with adapters

[kelly-sovacool]

add code to cutadapt rule. if the file is empty, throw an error