Add a try/raise block to catch exceptions from calling os module on None

If someone defines an Android target without specifying a 'manifest'
field in the BUILD file, they only got a cryptic error. The exception was
never called because the error came earlier, when calling os.path on a None
object. This ensures that the user sees the error message we mean them to.

I have added testing for Android targets to my TODO list.

This patch is small and shouldn't be dependent on any other open reviews.

Testing Done:
CI passes.
Did some fuzz testing manually.
Travis passes. [0]

[0] https://travis-ci.org/pantsbuild/pants/builds/32069220

Reviewed at https://rbcommons.com/s/twitter/r/860/