Add support for individualized error handling to the checkErr pattern

Right now we inject a checkErr handler, which we use for to cope with all err return where we always expect a nil response, and let the upper layer cope with it. But this doesn't allow for nuance in error handling. But in general for a CLI there is little nuance, if something generates and err, we just bail.

Comment in PR:

"Can the app layer implementations of checkErr similar to how there can be multiple levels of a try/catch for exceptions?
If there's only a global error handler referenced by checkErr then any code that needs individualized error handling isn't going to use this wrapper. "

Start by tracing how the injected checkErr handler is used across the CLI and how app-layer implementations could provide more specific handling. Define what individualized handling should look like alongside the global handler, then verify that errors without specialized handling still bail out as described.