miniswift: raise if no swift service is avail.
miniswift: raise if no swift service is avail.

In the SwiftClient class, it would seem kind of point to allow the
`auth()` process to look for a swift service, not find it, and then
carry on like everything is okay.

This change asserts that if there is no swift service available, we
raise a `ZPMException` immediately.

zerovm/zpm,zerovm/zpm,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zpm,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zpm,zerovm/zpm,zerovm/zerovm-cli,zerovm/zpm