gitweb: Jump do DONE_REQUEST not DONE_GITWEB on error

gitweb: Jump do DONE_REQUEST not DONE_GITWEB on error

On error (die_error()), instead of ending gitweb process by non-local
jump to DONE_GITWEB label at the end of requests loop in run()
subroutine, just end current request by jumping to DONE_REQUESt at the
end of run_request() subroutine.

This would allow to create new deployment mechanisms (for example
PSGI) based on run_request() and not on run().

Note that DONE_REQUEST label is removed because it is no longer used,
and it wouldn't work correctly for PSGI anyway.

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