Fail legacy install if .egg-info directory was not created

[chrahunt]

What's the problem this feature will solve?

Currently on the legacy install code path (here) if execution of setup.py install ... didn't result in an .egg-info directory being created we trace a warning and continue.

This is bad because we will have installed an uninstallable package and the only penalty is a small warning which is very easy to miss.

Describe the solution you'd like

Instead of tracing a warning we should throw an exception.

This prevents additional packages from being installed and possibly hiding the warning from users.

Alternative Solutions

Continue tracing the warning as before.

Additional context

• this support query for this package which does an egg install in its setup.py regardless of setuptools version.
• this query which implicates having an older version of setuptools

[pradyunsg]

Cool, let's give this the regular 2-release deprecation -- if no one raises major concerns w.r.t. this, we'll start raising an error there. :)