Fix python configuration processing.

Summary:
Previously buck would try to use any executable file or
directory in its working directory as a python interpreter. It would
also silently fall back to finding a python on the PATH if a python
path was specified in the config, but invalid. This change fixes both
of those problems and adds tests.

Test Plan:
0) buck test --all
1) mkdir python
2) touch BUCK
3) check that "buck build :lolwut" returns the error "BUILD FAILED: No rule
found when resolving target //:lolwut in build file //BUCK"