add optional QC check to filter_lastal_bam

[tomkinsc]

This adds an optional QC check to taxon_filter.py::filter_lastal_bam() to raise a QCError() if the sample name (bam file basename) begins with any number of negative control prefixes and lastal has identified reads to keep after filtering, where the number of reads is above a thresold of a minimum acceptable read count. This can be enabled via --errorOnReadsInNegControl. The readcount threshold can be set via --negativeControlReadsThreshold and the negative control prefixes can be set via --negControlPrefixes with defaults of "neg, water, NTC". The check has been added to the corresponding WDL file as well (off by default). A new errors.py file has been added to store error classes that may be useful across files.

If we end up liking this feature, the QC functionality can be abstracted into a separate class to inspect bam files and be included in various places in the codebase, along with an argparse base object that configures arguments for the QC checker.

[dpark01]

seems like a good & conservative way to add a new experimental feature we can play with.

Do we need to add the new errors.py to any of the scripts or files that list off all the top level py files (travis/coveralls/rtd scripts)? Maybe not...

[tomkinsc]

Not sure we need to add errors.py to the travis/coveralls/rtd scripts just yet; maybe once there's more functionality there than renamed base classes?