Bug 8007: (QA followup) Add error handling when generating the pdf

Bug 8007: (QA followup) Add error handling when generating the pdf

If error occurs when generating the pdf, it would be better to get an
encapsulated error instead of the "software error" message in the pdf
file.
To test this patch I added this change:

b/Koha/Borrower/Discharge.pm
-115,6 +115,7 @@ sub generate_as_pdf {
     say $html_fh $html_content;
     close $html_fh;
     my $pdf = PDF::FromHTML->new( encoding => 'utf-8' );
+    $html_path .= "poeut";
     $pdf->load_file( $html_path );
     $pdf->convert;

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>