catch missing chroms for mapped BAM alignments.  warn, don't out_of_range()
catch missing chroms for mapped BAM alignments.  warn, don't out_of_range()

jmarshall/bedtools2,jmarshall/bedtools2,arq5x/bedtools2,jmarshall/bedtools2,arq5x/bedtools2,arq5x/bedtools2,arq5x/bedtools2,arq5x/bedtools2,jmarshall/bedtools2,jmarshall/bedtools2