Update Regex util to support match limits

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.

headers/modsecurity/transaction.h

@@ -184,6 +184,8 @@ class TransactionAnchoredVariables {
         m_variableUniqueID(t, "UNIQUE_ID"),
         m_variableUrlEncodedError(t, "URLENCODED_ERROR"),
         m_variableUserID(t, "USERID"),
+        m_variableRxError(t, "RX_ERROR"),
+        m_variableRxErrorRuleID(t, "RX_ERROR_RULE_ID"),
         m_variableArgs(t, "ARGS"),
         m_variableArgsGet(t, "ARGS_GET"),
         m_variableArgsPost(t, "ARGS_POST"),
@@ -265,6 +267,8 @@ class TransactionAnchoredVariables {
     AnchoredVariable m_variableUniqueID;
     AnchoredVariable m_variableUrlEncodedError;
     AnchoredVariable m_variableUserID;
+    AnchoredVariable m_variableRxError;
+    AnchoredVariable m_variableRxErrorRuleID;
 
     AnchoredSetVariable m_variableArgs;
     AnchoredSetVariable m_variableArgsGet;