Enhancement: Verify that new or modified keys were stored correctly

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

We have added a check that the new key actually works before continuing. This
can protect against some (rare) cases of hardware or storage problems.

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