Make exit status on error same as bcftools

[jeromekelleher]

It's not clear that anyone would rely on the actual exit status values on error conditions, but it's something to watch out for. E.g.:`

$ bcftools query performance/data/sim_10k.vcf.gz 
Error: Missing the --format option
$ echo $?
255

vs

$ python3 -m vcztools query performance/data/sim_10k.vcz 
Usage: python -m vcztools query [OPTIONS] PATH
Try 'python -m vcztools query --help' for help.

Error: Missing option '-f' / '--format'.
jk@empire$ echo $?
2