Root all of our C++ exceptions to a common base exception

[mr-c]

lib/perf_metrics.hh:struct InvalidPerformanceMetricsKey : public std:: exception {
lib/khmer.hh:struct InvalidStreamBuffer : public std:: exception {
lib/khmer.hh:class khmer_file_exception : public std::exception
lib/read_parsers.hh:struct CacheSegmentUnavailable : public std:: exception {
lib/read_parsers.hh:struct CacheSegmentBoundaryViolation : public std:: exception {
lib/read_parsers.hh:struct InvalidCacheSizeRequested : public std:: exception {
lib/read_parsers.hh:struct NoMoreReadsAvailable : public std:: exception {
lib/read_parsers.hh:struct UnknownPairReadingMode : public std:: exception {
lib/read_parsers.hh:struct InvalidReadPair : public std:: exception {
lib/thread_id_map.hh:struct InvalidNumberOfThreadsRequested : public std:: exception {
lib/thread_id_map.hh:struct TooManyThreads : public std:: exception {

This is good from an API standpoint and helps us in the Python glue code

[iglpdc]

I'm working in this issue as part of the ScienceLab Sprint.

[mr-c]

Great. The common base exception should also be the base for khmer::khmer_file_exception

[mr-c]

This was completed in #554