[FIX] base: merging unreadable pdfs

To Reproduce
============
- create two Vendor Bills and attach to each one a PDF from the ones
provided by the client on the ticket.
- select these two bills and and try to print Original Bills an error will be raised

Problem
=======
while merging these PDFs, PyPDF2 throws a `TypeError` which is not caught by the server

Solution
========
catch `TypeError` to raise a UserError

opw-3285540

closes odoo/odoo#120756

Signed-off-by: abla001 <abla@odoo.com>