Meaningful message if corrupted setuptools

[idgserpro]

The primary mode of installation of setuptools is to use ez_setup.py: but if a beginner downloads a corrupted zip for some reason (check #557 for example) and don't delete the corrupted setuptools zip before running ez_setup.py again, the installation can't continue.

Since this is confusing to beginners the idea is to have a similar behavior like bootstrap-buildout.py: always download setuptools, but if you need for some reason to use a local file, use a parameter to have it (--setuptools-to-dir comes to mind).

ez_setup.py has now a specific branch and a test suite and this improvement should be done there.

[idgserpro]

@jaraco We think adding this option will add more confusion since ez_setup already has a "to_dir" parameter, which sets where setuptools should be downloaded as well.

In fact, if we change the behavior of download_setuptools in ez_setup, removing if not os.path.exists(saveto), it will break bootstrap-buildout.py since --setuptools-to-dir starts/set "to_dir" parameter, exactly for reusing an already downloaded setuptools.

We're not confident into changing this code. It has the potential of breaking other's people infrastructure.

SInce the motivation of this issue was

Shouldn't we have the same behavior in ez_setup.py? Always download but give the option to use a local copy, if needed? We're asking this because this "bad download" error is really confusing for beginners, since this is the official primary method of installation.

We started to think if we shouldn't just catch the zipfile.BadZipfile: File is not a zip file exception and add a message "Maybe you have a corrupt setuptools file. Remove it and run ez_setup again."

[jaraco]

Sounds like a simple and elegant improvement to me.

[idgserpro]

@jaraco Here we go: #842

[idgserpro]

Fixed #842