Added delegate method for failed system account access

This adds a hook to take action when a user attempts to share and they don't have the appropriate system account set up for the activity type.  Currently, OSK just presents alert messages.

Use case: We find many users don't have Facebook accounts configured in iOS, so we use this hook to fallback to sharing via Facebook's SDK (also mentioned here: https://github.com/overshare/overshare-kit/issues/10#issuecomment-33438239)

If you don't implement the delegate, the standard alert messages show.  It also returns the default action (showing the alert) if you want to perform a custom action for a certain activity type, but trigger the default for others.