Return a friendly error at flavor creation

When trying to create a flavor with an Id/Name that is already in use, the
operation fails and no error message is being displayed

Change-Id: I92db8b52bc4e63e5e5ce4d90169944e484baeddd
Fixes: bug #1079155
(cherry picked from commit dcda6940263a43344bf98aa1776cce34d25ff927)
Merge "Return a friendly error at flavor creation" into stable/folsom