Implement informative error messages for key errors

[pbrowne88]

When working with end-point classes in the pipeline (e.g. svo_bursts), users are often called upon to submit a filtering term (either an SVO or a token pair) for visualization steps.

Currently, if users submit an SVO or token pair that isn't present in the svo_bursts class, an uninformative KeyError will result. Consider replacing these with informative errors.