BiometricVaultException class sealed

The common base type for every failure this plugin reports.

The hierarchy is sealed, so a switch over an instance is exhaustive:

try {
  await file.read();
} on BiometricVaultException catch (e) {
  switch (e) {
    case AuthException(:final code):
      // The user could not or did not authenticate; inspect [code].
    case StorageInvalidatedException():
      // The stored value is gone for good; re-provision it.
    case BiometricVaultPluginException():
      // Unexpected platform failure; report it.
  }
}