log -L: improve error message on malformed argument

The old message did not mention the :regex:file form.

To avoid overly long lines, split the message into two lines (in case
item->string is long, it will be the only part truncated in a narrow
terminal).

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>