Better JIT error handling for patterns

[stevesmoot]

the regexp engine only supports a small number of "character class" patterns. If you try and use one
that isnt supported (for example the larger set binpac supported, you get a very obscure error response:
jitre internal error: ccl_from_std_ccl: JRX_STD_CCL_NONE givenAbort trap: 6

With absolutely no evidence I suggest the engine is probably giving us a better error that we could catch and make more descriptive. If not, perhaps just add to the debugging/FAQ.

[rsmmr]

Turns out it's actually the regex engine producing that and aborting. But we can fix that, I'll look into it.

printf 'xxx' | ./src/tests/retest '[[:xyz:]]'
=== Pattern: [[:xyz:]]
=== Data   : xxx

=== Standard matcher with subgroups
jitre internal error: ccl_from_std_ccl: JRX_STD_CCL_NONE givenAbort trap: 6

[rsmmr]

Fixed now in rsmmr/justrx#6, still needs a submodule bump.