wallet: clearer refusal for an empty or misnamed SQLite backend

Review follow-up on the SQLite activation. Two guards, both fail-closed;
neither is a correctness fix -- FinishLoadedWallet already refuses a
missing default key -- but they make the refusal say what happened:

- an empty or non-wallet wallet.sqlite is refused with "no default key
  record; it may be empty or not a wallet export", instead of the generic
  "no usable default public key" that reads as corruption;
- a misspelled -walletbackend value (e.g. sqlit) is refused instead of
  silently falling through to Berkeley DB; -walletbackend=bdb is accepted
  as the explicit default.

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