If logged-in, Post.url and Post.get_sidecar_nodes() now use the iPhone API
endpoint to obtain the highest-resolution image version. Resolves#630.
A notice "Warning: Use --login to download higher-quality versions of pictures"
is issued if the user tries to download posts without being logged-in.
Further, the message "Errors occurred:", which is displayed when the
InstaloaderContext is closed, has been changed to "Errors or warnings
occurred:".