Allow skipping files that do not exist during merge.

[donaldcampbelljr]

@nleroy917 Good idea. By the time the program gets this far, it has checked that the chromosomes exist in the bam file. However, it could raise an error during writing to the bed file line by line, but we should see this error in the terminal (which I don't in this case). We could skip that file. As long as the program communicates that it was expecting a chromosome file but didn't find one that would be fine. Currently, the messaging is inadequate either way as I'm not sure which chrom failed from the backtrace. :) Also, I would think the consumer handle would produce an empty file either way by this point.

Originally posted by @donaldcampbelljr in #74