`dump_rules` won't crash when rule failed to load dyna source.

`dump_rules` won't crash when rule failed to load dyna source.

Show failure context when an update handler crashes (work in progress)

when an initializer fails rules is still added to the program. Only some of the
emits which succeeded get scheduled (this isn't quite right).

added depth limit argument to trace (available via --limit=<INTEGER> at the
repl).