Explore graceful error handling for REPL

[Anton-4]

Crashes/panics during e.g. roc dev file.roc are not such a big deal, but in the REPL you've now lost all your state and you need to restart. We should think about non-crashing error handling for all code used by the REPL. We should still have an option to print backtraces on error because those can be useful in some cases.

[kukimik]

This may also be discouraging for newcomers. For instance, I wanted to experiment with the language in the online REPL (mainly with open/closed record types, as described in the tutorial) before taking the effort to actually install anything, but I experienced so many crashes that I gave up. I think I'll try the offline version, because I find the language interesting, but I imagine some potential users could drop out at this point.

[Anton-4]

Agreed!

[Anton-4]

A good first step here could be to produce nice error messages when something is not implemented, like in the todo's here. You can produce that panic by entering F = \x -> x % 3 in the (web) repl.