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.
On Mon, Jun 30, 2014 at 2:33 PM, Andrew Wilkins <notifications@github.com>
wrote:

> In version/supportedseries.go:
>
> > @@ -100,6 +100,8 @@ func SeriesVersion(series string) (string, error) {
> >  }
> >
> >  // SupportedSeries returns the Ubuntu series on which we can run Juju workloads.
> > +// Must only be run from the server side, not as part of the client as it
>
> Must it? We currently call it in the client when creating tools and image
> constraints for filtering simplestreams data. Are non-Ubuntu clients broken?
>
They must be. This test fails on osx, so the osx client must not be able to
perform this function.


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