Need to validate Manifest for existence of required Libraries property

[raquelsa]

We are getting null ref exceptions if Manifest does not have Libraries propertiy specified.
We should add validation for Manifest for required properties.

Trying to restore a LibMan.json without Libraries property will lead to the messages in the output window:

Restore operation started...
Restoring libraries for project Core20_AllRestoreErrors
Restore operation completed
Time Elapsed: 00:00:00.0062925
========== Finished ==========

[RobJohnston]

Duplicate of #249, created 3 weeks prior, tagged with "P3" and "bug".