Creates simple error handler to exit process.

[IonoclastBrigham]

Summary

So this seems to work, if perhaps somewhat ungracefully. It simply sets an error handler if we're not showing the gui, which prints the error and exits.

Caveats

1. I didn't totally get all the subtleties of the comments on Commandline invokations should fully exit with a non-zero status, on unrecoverable errors #81, so this doesn't do anything fancy besides just bail from the process altogether.
2. Probably a better way to print the error message; it's kinda ugly as-is

Feedback Welcome

Please!

[lehitoskin]

Instead of vomiting out the exception struct as-is, we could obtain only the message and print that out.

This might be helpful.

[IonoclastBrigham]

Niced up error message formatting with the latest commit. I didn't really grok what you were saying about using (exit:on-exit), so there's still nothing going on there. But I'm not sure we need a lot of explicit cleanup since we're totally killing the process here in a non-gui context?