SessionStorageFailure

Thrown when device storage itself could not be read from, written to, or deleted.

Deliberately distinct from "no usable session stored", which is reported as null: a caller must be able to tell "not signed in" from "couldn't read the Keystore".

The split is by what the caller can do about it. A storage layer that fails may succeed on the next attempt and says nothing about whether the user is signed in, so it is raised. A session that is present but unrecoverable — ciphertext whose Keystore key is gone after a backup or device-to-device restore, or contents that will not parse — will never become readable, so PortalAuth.restoreSession clears it and returns null instead of raising.