Give all exeptions thrown in read_parsers.cc a msg
Give all exeptions thrown in read_parsers.cc a msg

All exceptions in read_parsers.cc which don't have informative default
messages now have explicit messages in their ctor.

Winterflower/khmer,F1000Research/khmer,ged-lab/khmer,F1000Research/khmer,Winterflower/khmer,souravsingh/khmer,Winterflower/khmer,souravsingh/khmer,souravsingh/khmer,ged-lab/khmer,ged-lab/khmer,F1000Research/khmer