gitweb: Wrap die_error to use as error handler for caching engine

gitweb: Wrap die_error to use as error handler for caching engine

Use cache_error_handler subroutine, wrapping die_error (and
HTML-escaping error message), as 'on_error' handler for
GitwebCache::SimpleFileCache and its derivatives, and as both
'on_get_error' and 'on_set_error' handler for CHI based caching
engine.

Added single test in t9501 that checks if error in caching layer
produces "500 Internal Server Error".

Signed-off-by: Jakub Narebski <jnareb@gmail.com>