Handle truncated files more gracefully

[susannasiebert]

When a file is truncated pVACseq will error out with the following:

 File “/opt/conda/lib/python3.6/site-packages/lib/input_file_converter.py”, line 348, in execute
   genotype = entry.genotype(self.sample_name)
 File “/opt/conda/lib/python3.6/site-packages/vcf/model.py”, line 277, in genotype
   return self.samples[self._sample_indexes[name]]
IndexError: list index out of range

We should add a try except around this line and throw a more descriptive error message. Something like:
No genotype found for sample SAMPLE NAME at position CHROM POS. Is your file truncated?