Unknown --os or --arch still fetches the releases before failing #68

Description

An unknown --os or --arch value is only a warning (Possibly invalid operating system (--os=plan9) option). rr get then fetches the whole release list from the GitHub API and fails only afterwards with Could not find any available RoadRunner binary version which meets criterion. Each such call spends GitHub API requests (the anonymous limit is 60 per hour) on a request that can never succeed.

Expected: an unknown OS or architecture is rejected before any request, like an invalid --location already is.

Steps to reproduce

1. Run rr get --os=plan9 (or --arch=sparc).
2. The releases are requested before the command fails.

Additional context

Found by the acceptance tests in #66 (FailureTest::rejectsUnknownPlatformWithoutRequests). The release lookup may be replaced by DLoad in #49.