Enhancement: Verify that new or modified keys are stored correctly

When adding a new key or changing the password of a key, restic used to just
create the new key (and remove the old one, when changing the password). There
was no verification that the new key was stored correctly and works properly.
As the repository cannot be decrypted without a valid key file, this could in
rare cases cause the repository to become inaccessible.

Restic now checks that new key files actually work before continuing. This
can protect against some (rare) cases of hardware or storage problems.

https://github.com/restic/restic/pull/3429
