Datum Load configuration data fails silently when Data Handler Not available

[gaelcolas]

Seen when loading a Datum Tree without a defined Data handler.
The tree seemed to load fine, the DatumFileProvider looked fine, but the data was actually missing (i.e. $Datum.AllNodes.DEV.SRV01 was empty (but existing).

Validation should be done in New-DatumStructure to make sure everything will work as expected, or raise an exception and stop.