If the rx or rxGlobal operator encounters a regex error,
the RX_ERROR and RX_ERROR_RULE_ID variables are set.
RX_ERROR contains a simple error code which can be either
OTHER or MATCH_LIMIT. RX_ERROR_RULE_ID unsurprisingly
contains the ID of the rule associated with the error.
More than one rule may encounter regex errors,
but only the first error is reflected in these variables.
