gitweb: Use nonlocal jump instead of 'exit' in die_error

gitweb: Use nonlocal jump instead of 'exit' in die_error

Use 'goto DONE' in place of 'exit' to end request processing in
die_error() subroutine.  While at it, do not end gitweb with 'exit'.

This would make it easier in the future to add support or improve
support for persistent environments such as FastCGI and mod_perl.
It would also make it easier to make use of die_error() as an error
handler (for fatalsToBrowser).

Perl 5 allows non-local jumps; the restriction is that you cannot jump
into a scope.

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