GFF script: creates empty directory on error.

[julie-sullivan]

Dec 05 10:24:07 [DEBUG ] Not creating /micklem/data/human/gff/2018-12-05/ref_GRCh38.p7_top_level.gff3.gz as the downloaded file is empty
Dec 05 10:24:07 [INFO  ] New data available in /micklem/data/human/gff/2018-12-05
Dec 05 10:24:07 [INFO  ] Finished data download for NCBI Gene

An empty file should throw an error.

[vvijayalakshmi21]

Hi, I am an Outreachy applicant and would like to contribute to this issue.
Kindly assign this.