gitweb: More detailed error messages for snapshot format

gitweb: More detailed error messages for snapshot format

Improve error messages for snapshot format in git_snapshot:
distinguish between situation where snapshots are turned off, where
snapshot format ('sf') parameter is invalid, where given snapshot
format does not exist in %known_snapshot_formats hash, and where
gitweb was given unsupported snapshot format.

While at it, use first from all supported snapshots format as default,
if no snapshot format was provided.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>