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.
SupportedSeries() is indeed Ubuntu only - it is called when parsing simplestreams metadata when bootstrapping which is done from the client. It also assumes a Ubuntu workload since the purpose of the call is to vet the metadata associated with tools selection. The implementation could be tweaked for Windows and MacOS clients to use the hard coded series values and not error if the series cannot be updated from the ubuntu.csv file. But a lot more thought would be required for non Ubuntu workloads.