[5.x] Prevent published toast when save failed

[simonerd]

fixes #10190
replaces #10191

I've tried to stay as close as possible to the changes made in #9040 but needed some further adjustments. Specifically, Revisable::publishWorkingCopy() and Revisable::unpublishWorkingCopy() now return false if the entry save() function fails. While this seems reasonable to me, I haven't yet found out if there are any other places (apart from the PublishedEntriesController) that would need to be considered.

I have added translations for the "Couldn't (un)publish entry" toast message as far as my (non-german) language skills felt reliable enough.