FS-9741: lua expose db err str in freeeswitch.dbh

Added 2 methods to lua freeswitch.Dbh
`last_error()` Returns the error string from the last query or nil.
`clear_error()` clears the error string.

src/mod/languages/mod_lua/freeswitch_lua.h

@@ -58,6 +58,7 @@ namespace LUA {
   class Dbh {
     protected:
       switch_cache_db_handle_t *dbh;
+      char *err;
       bool m_connected;
       static int query_callback(void *pArg, int argc, char **argv, char **cargv);
     public:
@@ -68,6 +69,8 @@ namespace LUA {
       bool test_reactive(char *test_sql, char *drop_sql = NULL, char *reactive_sql = NULL);
       bool query(char *sql, SWIGLUA_FN lua_fun);
       int affected_rows();
+      char *last_error();
+      void clear_error();
       int load_extension(const char *extension);
   };
 }