Install command should install all or nothing

[njwilson]

Our current install command will happily install some packages and fail to install other packages. I think this is fine for now since we are just trying to get something working, but this should eventually change. We just need to clearly document the current behavior.

For example, if you do:

$ pip2 install a-real-package not-a-package

you will get

...
Successfully installed a-real-package.
Failed to install not-a-package.

If you do the same thing in the original pip, it will complain about not-a-package and nothing will be installed. Either everything gets installed, or nothing gets installed.