make UnsupportedPlatform more verbose

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
fail earlier when platform is unsupported in a given host

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
a wealth of tests for the better error reporting

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
Merge pull request #142 from alfredodeza/6915

better (earlier) error reporting for unsupported platforms