assert() may be compiled out in production and is clunkier to catch.
some ParserException are already thrown elsewhere in the code and it
seems to make sense to reuse the primitive, although it may still
crash improperly configured library consumers, those who do not handle
exceptions explicitly.
we use the BAD_FILE error message because at this point we do not
exactly know which specific data structure led to the recursion.
simply set a hardcoded recursion limit to 2000 (inspired by Python's)
to avoid infinitely recursing into arbitrary data structures
assert() the depth. unsure if this is the right approach, but given
that HandleNode() is "void", I am not sure how else to return an
error. the problem with this approach of course is that it will still
crash the caller, unless they have proper exception handling in place.
Closes: #459
