check for errors on iteration

[brentp]

I've hit this in other libraries as well. sam_read1 and most iterators in htslib return -1 for end of iterator and < -1 on error. It's important to know the difference. Otherwise, on a corrupt bam or network error, it will not detect an error it will just stop iteration.

This problem is seen here:

HTSLibXX/include/BamReader.hpp

Line 41 in 7873c9b

if (sam_read1(file.fp, _hdrs[file.idx].GetHeaderStruct(), rec_ptr) >= 0)

[38]

So when it's an error we should set the error string of this BamAlignment.

Returnting false on either error or EOF is the behavior of BamTools.

Currently I haven't implemnet the error string yet. But this is a good note for we should add the error string for the error cases later.