cgit: override die routine globally

We don't get any return value from compile_grep_patterns calling
compile_regexp_failed, causing the default die routine to print to
stderr and then for cgit to exit ungracefully.

Instead override the default die routine to show a normal error page.

Perhaps compile_grep_patterns ought to change upstream to return an
error. But this commit here will handle future issues as well, so
perhaps not a bad idea to do anyway.

Link: https://lists.zx2c4.com/pipermail/cgit/2026-March/004982.html
Link: https://lists.zx2c4.com/pipermail/cgit/2026-March/004983.html
Reported-by: Adrian C. <anrxc@sysphere.org>
Reported-by: Aiden Woodruff <aiden@aidenw.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>