Bug 26048: Use ErrorDocument middleware for Plack HTTP errors

Bug 26048: Use ErrorDocument middleware for Plack HTTP errors

This patch uses the ErrorDocument middleware to use Koha's
custom error pages instead of generic Plack error responses

Test plan:
0. Apply patch
1. vi /usr/sbin/koha-plack (and change "development" to "deployment")
2. vi ./opac/opac-main.pl
3. Add "die" to line 2
4. vi ./mainpage.pl
5. Add "die" to line 2
6. cp ./debian/templates/plack.psgi /etc/koha/sites/kohadev/plack.psgi
7. koha-plack --restart kohadev
8. Go to http://localhost:8080/cgi-bin/koha/opac-main.pl
9. See a beautiful OPAC 500 error instead of "Internal Server Error"
10. Go to http://localhost:8080/cgi-bin/koha/blah.pl
11. See a beautiful OPAC 404 error instead of "not found"
12. Go to http://localhost:8081/cgi-bin/koha/mainpage.pl
13. See a beautiful Staff interface 500 error instead of "Internal Server Error"
14. Go to http://localhost:8081/cgi-bin/koha/blah.pl
15. See a beautiful Staff interface 404 error instead of "not found"

For bonus points:
16. koha-plack --disable kohadev
17. koha-plack --stop kohadev
18. service apache restart
19. Repeat the above test plan to show CGI still works for 404 (although 500
will show "Software Error" due to C4::Context needing some improvements)
20. Using the "Network" tab on your developer tools, make sure 404 and 500
are returned by the appropriate error pages

Signed-off-by: Eden Bacani <eden.bacani@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>