Translations silently fails if in preview mode

[cdrini]

Clicking translate for a book that hasn't yet been borrowed causes the translation to silently fail, and spin forever. Instead should alert an error.

To reproduce:

1. Go to a previewable book, eg https://archive.org/details/isbn_9780316253017
2. Click "Translate"

[miantalha45]

Assign it to me, i want to work on it.

[Himanshu4812]

Hi! @cdrini
I’ve submitted a PR #1497 that fixes this issue #1489 by adding a null guard for missing .BRtextLayer and showing a user-facing message for preview-only books.
Could you please review it when you get a chance? Thanks!