On attaching from remote, be sure we don't create a message that's not

On attaching from remote, be sure we don't create a message that's not
allowed to send because of its size

* src/modest-ui-actions.c (modest_ui_actions_compose_msg): now we check
  the expected size of the message even before showing the editor. If
  message is too big we return an error, and tell we cannot create the
  message. We also check now if the message is bigger than the available
  disk space.