[telepathy-sasl-signon] Don't set CredentialsNeedUpdate on global service

Instead, set it on the service which we're using for the account.
Also, print a debug line to the log when raising the flag.
[telepathy-sasl-signon] Only set CredentialsNeedUpdate flag when appropriate

Previously, the CredentialsNeedUpdate flag would be set in a variety
of situations, including when a generic OperationFailed error occurred.
This commit ensures that it is only set for an account when a
UserInteraction error occurs (that is, the account credentials are
valid but need to be refreshed manually by the user).
Merge pull request #15 from chriadam/cnufixes

Fix some issues with CredentialsNeedUpdate handling