Add warning when running sage -i

[slel]

Since the app does not ship enough to build extra packages, consider having sage -i blah warn that this is not supported, instead of erroring out.

Some reports from users who tried to install extra packages:

• sage-support discussion: sage -i gap_packages
• Ask Sage question 57450: sage -i pynormaliz

[culler]

This suggestion has been implemented in v1.2.1, so I am closing the ticket.