version: fix package and tests

Introduce `mustOsVersion`, which replaces `osVersion`, and panics if the version cannot be determined.

`osVersion` now returns an error value if it cannot determine the series.

Fix the tests so they actually _compile_ and _pass_ on OSX.
Must it? We currently call it in the client when creating tools and image constraints for filtering simplestreams data. Are non-Ubuntu clients broken?