Error recovery

[Techcable]

This is a nice-to-have feature, which isnt very useful in the context of an interpreter.

I think the only technique I would reasonably use is panic mode.

Recovering from mismatched tokens and lexer errors would be hard (especially with logos), but other types of unexpected tokens should be easier.

We should also consider switching entirely to aridane for errors, although that's really a separate issue.