Bug 14103: PDF::FromHTML is conditionally required on UseDischarge systempreference

Bug 14103: PDF::FromHTML is conditionally required on UseDischarge systempreference

Since this is only in generate_as_pdf of Koha::Borrowers::Discharge, it made
sense to change it from a use to a require (avoiding a compile time error),
and placing it inside the eval where the PDF is generated, thus capturing the
problem in the koha error log.

TEST PLAN
---------
1) Log in to staff client
2) Koha Administration -> Global system preferences -> Local Use
3) Lower the Version number by 1, but DO NOT CLICK SAVE.
4) At the command line, sudo apt-get purge libpdf-fromhtml-perl
5) Back in the staff client, click save.
6) Every looks great until you attempt to log in. KABOOM!
7) Apply patch
8) Log in to staff client
   -- YAY!
9) Koha Administration -> Global system preferences -> Patrons
10) Make use the useDischarge is set to Allow
11) Find a patron with nothing checked out.
12) There is now a Discharge tab on the left at the bottom.
13) Click the 'Generate Discharge' button.
    -- nice error message
    -- check koha error log file for the ugly @INC missing message
14) At the command line, reinstall PDF::FromHTML
    (I built my own with cpan2deb: sudo dpkg -i /path/to/libpdf-fromhtml-perl_...)
15) In the staff client, click the 'Generate Discharge' button again.
    -- PDF is created as expected. All is well.
16) run koha qa test tools.

Signed-off-by: Chris Nighswonger <cnighswonger@foundations.edu>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>