differantiate error codes from h_compile() and update documentation

the regex backend already returned 2 for an internal failure, contrary to
documentation, so this mainly brings the latter in line with existing behavior.

at the same time, fix a bug: h_compile would previously return "false" (0) on
exceptions (longjmp).

make use of our new freedom to signal different error conditions from the
context-free backends, notably "incompatible combinator" vs. "unresolvable
conflict".