Add custom exceptions

[nebfield]

This module defines a custom PGS exception hierarchy. There's a lot of exceptions for specific failure states,
which can be a bad approach and too complex. However, we did this anyway for a few reasons:

1. There's only a few types of common errors (around a dozen, with 3-4 very common)
2. Want to exit the program with custom exit codes to simplify communicating program
   state with external processes (e.g. PGS Catalog Calculator) without doing
   complicated things like logging to an external location
3. This approach should make maintaining exit codes simple

So the plan is to override sys.excepthook, intercept errors defined here, and map them
to custom exit codes defined below

[smlmbrt]

I don't see any problems with this, but it might make sense to organise the code to be:

Base Class
CatalogError
Specific catalog error
MatchError
Specific MatchError
...