fail more gracefully when setuptools is downlevel (#1975)

[beckjake]

Fixes #1975

When we can't import find_namespace_packages, print a message and exit with a non-zero code.

Before:

$ pip install -r requirements.txt
Processing ./core
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/private/var/folders/31/mrzqbbtd3rn4hmgbhrtkfyxm0000gn/T/pip-wc7b1oy9-build/setup.py", line 2, in <module>
        from setuptools import find_namespace_packages
    ImportError: cannot import name 'find_namespace_packages'

Now:

$ pip install -r requirements.txt
Processing ./core
    Complete output from command python setup.py egg_info:
    Error: dbt requires setuptools v40.1.0 or higher.
    Please upgrade setuptools with "pip install --upgrade setuptools" and try again

[drewbanin]

some minor copy comments here, nothing major. One quick question: is there any merit to stating a requirement on the version of setuptools? Or is that not appropriate here?

[beckjake]

Unfortunately, this error happens before pip gets to do things like process install_requires, so I don't see how it will help.

[drewbanin]

^ that makes perfect sense, thanks for clarifying!