Fix wrapping cmake

1. Addresses #41.
2. Improves error handling for Python wrapping.
3. Attempts to fix an issue with building the wrapping on a mac (see https://simtk.org/forums/viewtopic.php?f=91&t=4969&amp;sid=a698723064ebd9b1d2e5aae9fc4ff0f0)
4. For the SWIG java target JavaWrap, only rebuild if out of date (previously, this target was always out of date).

I don't think this is ready to go in yet; I think I want to do some of my own testing first.

This used to be PR #42. I had to create a new PR b/c that PR was from my fork to this repo. When this repo went public, the "fork" connection seems to have been destroyed.

Anyway, this is ready to go in. @aymanhab , can you review?