Remove the account error scrollbook from the blist as notifications have replaced it.

Also clear account errors when we attempt a new connection which made clearing
the errors after the account has connected redundant.

Testing Done:
Caused the same account to get disconnected between pidgin 2 and 3 to verify notifications were working as expected.

Bugs closed: PIDGIN-17640

Reviewed at https://reviews.imfreedom.org/r/1521/