gtkplacessidebar: Improve error message when unlocking volume fails
A user test showed that the error message created by the places sidebar was not helpful to the participants:
https://mailman.boum.org/pipermail/tails-ux/2018-June/003554.html
We hope that this small patch can improve the UX a bit. The better solution would be to display in the error message (and the title) why exactly the unlocking failed - but this is much more complex, as it requires significant changes to how libcryptsetup handles errors.
Corresponding gtk 3.24 merge request: !266 (merged)