gitweb: improve snapshot error handling

gitweb: improve snapshot error handling

The last check in the second block of checks in the &git_snapshot routine
is never executed because the second to last check is a superset of the
last check.

Switch the order of the last two checks. It has the advantage of giving
clients a more specific reason why they cannot get a snapshot format if
the format they have chosen is disabled.

Signed-off-by: Mark Rada <marada@uwaterloo.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>