extmgr: raise config exception if requested extension is null

Topic: AAA
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1139671
Change-Id: Ie32e829d4a8c0790b2769169e0c8c793d4d71ba3
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>