Bug 14574: Fixing error messages on staff client so they make more sense

Bug 14574: Fixing error messages on staff client so they make more sense

To test:
1) Go to cgi-bin/koha/errors/errornumber.pl to see the error messages which currently don't make sense (ie cgi-bin/koha/errors/401.pl etc). Have only made changes to 401, 403 and 404
2) Apply patch
3) Go to cgi-bin/koha/errors/401.pl --> Check that it now says "This error means that you aren't authorized to view this page." and "Please log in and try again.". Check that 'log in' redirects to the log in page (mainpage.pl).
4) Go to cgi-bin/koha/errors/403.pl --> Check that it now says "This error means that you are forbidden to view this page."
5) Go to cgi-bin/koha/errors/404.pl --> This page should be very different. Check that the 'Error 404' is now in italics and not bold. Check that the page gives a list of reasons the user may have been given this error and some options for their next step.

Signed-off-by: Catherine <cnorthcott.work@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Amended patch: replace tab with 4 spaces.
Note: I am not sure the em tag is useful here.
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>