Double check for failed cache with a shared storage

When obtaining the SourceRootInfo we can call init. If this fails the
cache is set to a failed cache and the storage to a failed storage.
However we did not check for this. Which means that if the storage was
invalid it would fail later on.

Now we will properly error out.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>