detect when setuptools is downlevel and abort the isntall with a good error

[beckjake]

Describe the feature

dbt requires a version of setuptools greater than v40.1.0 to install. We should check for this version and raise a useful error message if the user is downlevel.

Describe alternatives you've considered

We can just ignore this and let users get errors about not finding find_namespace_packages at install time.

Additional context

dbt requires the find_namespace_packages function from setuptools, which was introduced in v40.1.0 (see here). We currently crash with a pretty unhelpful error message about cannot import name 'find_namespace_packages'. Instead we should check and raise an error instructing users to run python -m pip install -U seuptools.

Who will this benefit?

Users trying to install dbt, especially those new to python/pip.

[drewbanin]

really good idea @beckjake - is this something we can hit for 0.15.1? I just put it in that milestone, but we can change to 0.15.2 if needed

[beckjake]

Yeah, totally! It should be a trivial change, just a quick tweak to setup.py to catch the ImportError I would guess. If it's harder than that we can kick it out.