Thanks for looking and for the suggestions, Michael.
Yeah, this was also brought up in a question I asked a few days ago (Ergonomics of a lexer and iterators - #8 by jmmv). At that point, I spent some time removing the use of fallible from the reader and the lexer, but then stopped there given that, before changing semantics, I wanted to get the whole thing fully tested (even if with suboptimal behavior).
So yes, I want to make error reporting better (which will be necessary for recoverability in the REPL). It currently sucks right now 
I think I'd also want to avoid using fallible completely, but we'll see how far I take that.
Cheers!