gitweb: standarize HTTP status codes

gitweb: standarize HTTP status codes

Many error status codes simply default to 403 Forbidden, which is not
correct in most cases.  This patch makes gitweb return semantically
correct status codes.

For convenience the die_error function now only takes the status code
without reason as first parameter (e.g. 404 instead of "404 Not
Found"), and it now defaults to 500 (Internal Server Error), even
though the default is not used anywhere.

Also documented status code conventions in die_error.

Signed-off-by: Lea Wiemann <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>