Create and add notifications for account errors.

Adjust the NotificationManager API a bit to be easier to use and map better to
GListStore which the manager is now using internally.

Testing Done:
Ran the unit tests and forced a connection error and verified no issues (used gdb to verify that item was added).

Bugs closed: PIDGIN-17639

Reviewed at https://reviews.imfreedom.org/r/1509/