provider:checkbox: quit earlier when no eligible disk are found

This patch makes removable_storage_test quit eariler when no disks are
eligible to run the test.

Fixes: https://bugs.launchpad.net/plainbox-provider-checkbox/+bug/1382346
Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>
"automatic merge by tarmac [r=taihsiangho,zkrynicki][bug=1382346][author=kissiel]"