Find and eliminate upstream S-expression exception printer

[aantron]

...which sometimes clobbers exception logging output, especially since it itself sometimes raises exceptions as part of its control flow.

Don't have an example ready ATM as I last observed this before I was using issues in the repo, will update later.

• [ ] In general, look through Dream and deps for exception-based control flow and get rid of as much of it as possible.

• [x] EDIT: it may be possible to work around this by saving the backtrace before calling Printexc.to_string.