To add explicit handling on missing DS in CYVCF2Genotype

[liangyy]

When user wrongly specified --vcf_mode imputed but there is actually no DS field in VCF, the script will silently exit due to the KeyError exception in the generator at this line:

MetaXcan/software/metax/genotype/CYVCF2Genotype.py

Line 61 in 122e5fd

d = numpy.apply_along_axis(lambda x: x[0], 1, variant.format("DS"))

In order to improve the error messaging, we need to handle the exception explicitly by raising up the error.