--stop doesn't really work

[tsani]

The --stop option was added to Harpoon to assist in debugging command transcripts, to make Harpoon exit on the first command that produces an exception. The problem is that errors that are generated by Harpoon rather than by Beluga do not generate exceptions. They just print an error message.

This is tagged refactor because one should rethink the way exceptions are handled in Harpoon. Perhaps a simple approach is to make all errors proper OCaml exceptions. Since these exceptions have no need to be caught and analyzed, we can just wrap a function of type formatter -> unit -> unit that prints the error message.

[tsani]

Given that --stop is largely a debugging tool, I've made this issue low-priority for now.