Add PurpleAccount::error property and remove "account-error-changed" and "account-status-changing" signals.

Also sorted the account signal changes in ChangeLog.API.

The account-error-changed signal is easily replaced by the notify signal for the
new property, and account-status_changing wasn't used by anything.

Testing Done:
Used the account actions of the demo protocol plugin to disconnect the account without issue.

Reviewed at https://reviews.imfreedom.org/r/2036/