Add a more straightforward way to reset crashed pages

[olifre]

Very happy with this app, many days of my life already spent using it!

Doing something similar to #1282 (and opening a book right before the loaded book was rendered), I managed to crash LibreraReader and ended up with a page marked #crash without a way to reset this.

Checking the sources, it seems this is done by this commit:
12ec95b

Even closing / reopneing the book, force-closing Librera, resetting read progress, rebooting the phone etc. did not reset this crash marking of the page.

Checking further, it seems that currently, the only way to reset this is to trigger SearchAllBook by going into the library and clicking "refresh" there to reindex all book metadata, which I have used to successfully reset the crash marking so I can read the affected book in full again.

It would be nice if there was a ,more straightforward way to trigger ImageExtractor.clearErrors() and thus reset those crash markings ;-).