Check for gzopen() failure and print error messages

[jmarshall]

Ensure that all instances of gzopen() have appropriate error checking. In a couple of cases, lift the gzopen to above some allocations so that the error path doesn't need to undo the allocations. Down in misc/*.c the infrastructure is lacking and we don't go to great lengths to e.g. return a non-zero status.

Fixes another few instances of #51.

Noticed in bedcov by @rotifergirl (see https://twitter.com/Julie_B92/status/1097867250642628608).

[daviesrob]

Thanks. More work needed here, but it's a start.

Bedcov getting into an infinite loop suggests something lacking in kseq too.

[jmarshall]

Yes, kseq and kstream basically have no error checking — IMHO the way to deal with this is to exterminate places where they're used. See also samtools/htslib@71c03b8's commit message.

I'm sure bedcov.c has better ways to read BED files nowadays, but this was the small quick fix…