[FIX] base: catch PDFs that fail on PdfFileWriter.write

Issue:
Our functionality for merging PDFs from multiple vendor bills relies on PyPDF2. It is well-known that PyPDF2 is sometimes unable to manipulate even perfectly normal PDFs. To provide a helpful error message to the user when this happens (i.e. give them the names of the vendor bills corresponding to the offending pdfs), the `_get_unreadeable_pdfs()` function is called right before we try to merge the PDFs in `_merge_pdfs`. This function is meant to identify the offending PDFs and provide them to the user.
However, _get_unreadable_pdfs did not notice the problem with 3 of my customer's pdfs, because the error was only triggered once the PdfFileWriter.write function was called in _merge_pdfs. This function, however, is not called in _get_unreadable_pdfs, therefore, it did not notice that anything was wrong.

Fix:
- Change _get_unreadable_pdfs so that it also makes a call to PdfFileWriter.write
- As soon as PdfFileWriter.write fails once, it will continue failing when we append more PDF streams to the PdfFileWriter. I therefore suggest initialising a different PdfFileWriter at each iteration of the for loop in order for an offending PDF to not cause a false positive on subsequent PDFs.

closes odoo/odoo#78905

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>