ref-filter: add return value && strbuf to handlers

ref-filter: add return value && strbuf to handlers

Continue removing die() calls from ref-filter formatting logic,
so that it could be used by other commands.

Change the signature of handlers by adding return value
and strbuf parameter for errors.
Return value equals 0 upon success and -1 upon failure.
Upon failure, error message is appended to the strbuf.

Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>