every step of error handling. Compared to Frédéric's patch, this
code is "more incremental" in that the parser will try to handle
the error, pop one cell, suspend itself, try to handle the error,
pop one cell, suspend itself, etc. In other words, *every call*
to [error env] has been replaced with [HandlingError env], whereas
in Frédéric's patch only 2 out of 3 calls were replaced.