Blank lines in fasta don't have a useful error message

Blank lines in fasta files seems to be a fairly common failure mode, but the error message is just:

```
  File "/Users/maurocalabrese/anaconda3/lib/python3.7/site-packages/seekr/fasta_reader.py", line 53, in _upper_seq_per_line
    if line[0] == '>':
IndexError: string index out of range
```
It'd be nice to report what line number is blank so the user doesn't have to go searching.