Print an error if filename is not found in html_include.

Normally when html_include cannot open the file it fails silently and
things can be a bit hard to figure out from just looking at apache's
log. This will be beneficial for those initially setting up their server
with cgit.

Signed-off-by: Harley Laue <losinggeneration@aim.com>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>