Improve error message when the default role doesn't exist

To add members to a project, Horizon requires a default role to exist
in Keystone.  This provides a clearer error message when the role
cannot be found.

Fixes bug #1108063

Change-Id: I0e3cd4cb96c7896c675055671e0875840b1a41d3
Merge "Improve error message when the default role doesn't exist"