Throw a proper error if the flavor is missing.

I ran into a bug which caused a bt when the flavor is missing on the
targetted system.  This patch makes it throw a proper exception and a
useful error message.

Fixes: bug #1129388
Change-Id: I545fce32732522053e2a4a113a02883a89045910
Signed-off-by: Ian Main <imain@redhat.com>
Merge "Throw a proper error if the flavor is missing."