BUGFIX: Proper error if site node not found in backend

[mhsdesign]

It seems the (internal) contract for providing the UI state requires no site or document node, but this is not true (obviously)
The defaultNodesForBackend helper will just crash:

defaultNodesForBackend(): Argument #1 ($site) must be of type Neos\ContentRepository\Core\Projection\ContentGraph\Node, null given

It is invoked here via YAML - hence we did not detect this null pointer bug:

neos-ui/Configuration/Settings.yaml

Line 131 in ebed351

byContextPath: '${Neos.Ui.NodeInfo.defaultNodesForBackend(site, documentNode, request)}'

Instead now a pretty error is throw which will also highlight probably its cause:

Site node "neosdemo" not found in content repository "default" in dimension space point {"language":"en_US","targetGroup":"private"} and visibility constraints "removed"

In my case i had a configuration laying around adding a new dimension targetGroup for all sites which was un nice which lead to no nodes being found.