VCF import - check that success count + failure count = input count

BCFTools separates variants into good (which are written into the output.vcf) and rejected (written to reject vcf)

We should run a check that the number of records in both of these equal the input, to double check that none have gone missing

Noticed this during issue #1079 - where I had to remove check that success VCF had non-zero entries (as that's a legit result that everything went to reject pile)