scrypt error rework

[simonsan]

rustic_core/crates/core/src/repofile/keyfile.rs

Line 109 in 25f908b

"Invalid scrypt parameters. Please check the key file and password.",

Error needs own error code, with link to docs.

old restic versions used invalid scrypt parameters which are not (and I think will never be) supported by rustic

proposed workaround is to recreate the key in restic

Ref.: rustic-rs/rustic#1394

[aawsome]

As written in rustic-rs/rustic#1394 I propose to wait with this until we get a decision from the Rust Crypto team about how they'll treat this. As there is already a RFC errata and this issue also occurs in e.g. etherium implementations, IMO there is a good chance this check will be relaxed in upstream and we won't see this error anymore in future.