Add error message if assign roles with restrictions
Add error message if assign roles with restrictions

Adds more verbosity to error message of Nailgun response
during assigning nodes roles with restrictions

Change-Id: Ibccd0bf0bec2097fdebd08bc4565c06c15d88135
Closes-Bug: 1591447

stackforge/fuel-web,stackforge/fuel-web,stackforge/fuel-web