gitweb: Die if there are parsing errors in config file

gitweb: Die if there are parsing errors in config file

Otherwise the errors can propagate, and show in damnest places, and
you would spend your time chasing ghosts instead of debugging real
problem (yes, it is from personal experience).

This follows (parts of) advice in `perldoc -f do` documentation.

This required restructoring code a bit, so we die only if we are reading
(executing) config file.  As a side effect $GITWEB_CONFIG_SYSTEM is always
available, even when we use $GITWEB_CONFIG.

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