In the beginning I wasn't sure if I perhaps should make errors return
a value (e.g. undef, or an error object of some kind) or let it throw
an exception. it was the latter then, but there were many remains of
the former ("return error(...") everywhere. That didn't matter a lot,
but over the time had become a nuisance.

And then there were notions of catching and somehow acting on the error
before passing it on that were unneeded in between or simply wrong. All
these have been straightened, and errors are now caught only in places
where things must be restored to a previous state (let, labels, eval_form)
or where a traceback shall be printed.