misc changes
incorporate ability to check for finished sequences (but not the actual quitting)
took out checks for modules, as we don't use any anymore
Added ability to specify a particular filename for the blast_file (instead of always using a tempfile)
If the file *ends* in q, not just contains "fastq"
made function "exit_with_msg" to handle details of quitting unexpectedly.
should only open the CONTIGS_FH file right before writing to it, then should close.
Die if blastn doesn't find any reads.