friendlier setup.py errors when numpy is missing.

[gregglind]

(I have a patch for this coming in, see if you like where it goes)

issues:

1. need numpy for dependency compilation
2. no easy way to tell setup.py to get numpy and work with it first. (or is there?)
3. $ pip install numpy pandas still isn't enough.

So the best I can do I give a friendlier message. Patch inbound.

[wesm]

This looks good to close