python2: Don't fail silently on module build.

A build output of such a failure can be found here:

http://hydra.nixos.org/build/21994789/nixlog/1/raw



The build_ext command doesn't return non-zero if it fails but instead
produces a file with a "_failed.so" suffix, which we're now checking
after the build_ext command.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>