The kerberos provider was marking all errors as authorization
failures resulting in false positives in many situations.

Rely on the GoaIdentityManager to return the correct errors, and
act on those that imply a real error occured.

closes #367

If the requested session keyring alias doesn't exist, the call to
retrieve it returns NULL without setting the GError.

Detect this case and return the best user-visible string we can
without breaking string-freeze, and log the real error as a
warning.

closes #376

GNOME 47 Backports

See merge request GNOME/gnome-online-accounts!285

Fix two arguments swapped in a signal disconnection, resulting in
a use-after-free error.

closes #386

GNOME 47 backports

See merge request GNOME/gnome-online-accounts!297