An invalid ISBN causes the book page to crash (quite problematic)

[prolibre]

On bookwyrm 0.7.5

Describe the bug
An invalid ISBN causes the book page to crash.

To Reproduce
All you have to do is edit a book and enter this isbn: 97801X45 (please note that you then have to correct this directly in the database).

Following a server-side error on a book, I looked at the logs and saw a line like :

Apr 29 09:57:02 master gunicorn[1647]: File "/DIR/bookwyrm/isbn/isbn.py", line 122, in __find_registrant
Apr 29 09:57:02 master gunicorn[1647]: if registrant_range[0] <= int(registrant) <= registrant_range[1]:
Apr 29 09:57:02 master gunicorn[1647]: ^^^^^^^^^^^^^^^
Apr 29 09:57:02 master gunicorn[1647]: ValueError: invalid literal for int() with base 10: '1X'

For some reason the book's ISBN was wrong. The value 97801X45 caused the page to crash.