specific client exception types

[pnwamk]

Currently the Haskell server backend uses specific error codes to describe different kinds of errors, see here.

The argo-client however uses a single class ArgoException for all errors. It may be useful for client applications if we have some subclasses of ArgoException to classify errors which a client may wish to react to, e.g., perhaps a timeout shouldn't crash an entire script, but instead be caught and count as a single test failure in a larger script with many tests.