Gracefully handle another run creating latest link

Gracefully handle a race with another run creating or removing the latest link (which can happen with concurrent runs).

Testing Done:
Concurrent runs ~8µs apart.
https://travis-ci.org/pantsbuild/pants/builds/41989058

Bugs closed: 827

Reviewed at https://rbcommons.com/s/twitter/r/1396/
Sync of https://github.com/pantsbuild/pants/commit/a453ca8ccbdcc08f3f530abb878ea2b9484745e3