Make Subprocess::spawn more robust

Make Subprocess::spawn more robust

Summary:
We can't throw after the process is created, because we don't know what to do
with it (and the Subprocess object goes up in smoke, so we can't rely on the
caller to clean up, either).  So don't throw.

If we throw before the process is created, make sure we clean up.

Test Plan: subprocess_test

Reviewed By: delong.j@fb.com

FB internal diff: D774722