[FIX] Hide the error banner if no error has occured. [#573 state:fixed]

- Added a hook into -[BannerController updateBannerForViewingState:] since that's
  where Mail.app actually decides if an error banner should be shown or not.
- Check if an error banner is already shown first, since there are various
  kinds of banners, like JunkBanner, CalendarBanner and others.
  If no banner is shown, display the certificate banner only if the viewingState
  has an error set.
- Remove [MessageContentController MA_backgroundLoadFinished] which is replaced by
  -[BannerController MAUpdateBannerForViewingState:]
- Remove the shouldShowErrorBanner property from Message.
  It's no longer necessary.