CA-182929: Raise an error if the storage backend type is unknown.
CA-182929: Raise an error if the storage backend type is unknown.

Previous behaviour was to wait for a storage backend of the named type
to be registered.

Signed-off-by: Jon Ludlam <e7e3380887a8f95cc9dc4f0d51dedc7e849a287a@citrix.com>

euanh/xen-api,Frezzle/xen-api,thomassa/xen-api,srowe/xen-api,euanh/xen-api,Frezzle/xen-api,euanh/xen-api,Frezzle/xen-api,srowe/xen-api,srowe/xen-api,thomassa/xen-api,Frezzle/xen-api,srowe/xen-api,thomassa/xen-api,euanh/xen-api,srowe/xen-api,Frezzle/xen-api,thomassa/xen-api,euanh/xen-api,thomassa/xen-api,thomassa/xen-api,euanh/xen-api