If ePrimer3 does not generate primers, the pipeline hangs

[widdowquinn]

While looking at P. capsici data, the eprimer3 step can generate .eprimer3 files with no contents. This is not currently detected as a problem by pdp, and trying to process an empty file hangs the pipeline.

Proposed solution: detect empty .eprimer3 files and report an error.

Another option: also require that sequences being stitched are longer than the proposed length of primers, and add an option to --fix_sequences.