"Key storage out of sync" flow gives misleading error message when 4S lacks the secret

[richvdh]

Given an account where:

• 4S is set up (i.e., a configured m.secret_storage.default_key)
• but 4S does not contain the master key (m.cross_signing.master is missing or empty)
• EX is missing the private master key

(This is quite possible on older accounts: once upon a time, we did not store the master cross-signing key in 4S)

As expected, we see the "Your key storage is out of sync" toast. Click "Continue", enter the correct recovery key. Observe "Incorrect recovery key":

[richvdh]

Related: matrix-org/matrix-rust-sdk#4736