Generate help text automatically from error_codes

[AbstractBeliefs]

We use error codes for signalling different kinds of failures from the tools - these error codes are all listed with symbolic and numeric values.

It would be good if the help text quickly ran off a list of what error codes existed and what they meant. We should be able to do this automatically rather than having to keep the help text and code list in sync.

[mayacopeland]

Could create a custom exit function?
Would work but would maybe create bloat.