MINOR: lua: Slightly improve function dumping the lua traceback

MINOR: lua: Slightly improve function dumping the lua traceback

The separator string is now configurable, passing it as parameter when the
function is called. In addition, the message have been slightly changed to
be a bit more readable.

(cherry picked from commit d09cc519bd0c04a0d1ec5bf23ed053c1e596851a)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 5326d6a152e7001c325ce3b00636132f0fb419eb)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit c4e150a99d5eb31ab7fd6355b9f21060b9433ccb)
Signed-off-by: Willy Tarreau <w@1wt.eu>