cmd/dex: validate that connectors have an ID.
cmd/dex: validate that connectors have an ID.

Calpicow/dex,coreos/dex,Calpicow/dex,jwmaag/dex,chub/coreos-dex,wolfeidau/dex,circleci/dex,holgerkoser/dex,ericchiang/dex,jwmaag/dex,holgerkoser/dex,rithujohn191/dex,rithujohn191/dex,coreos/dex,circleci/dex,Calpicow/dex,xeonx/dex,cjyar/dex,xeonx/dex,wolfeidau/dex,rithujohn191/dex,xeonx/dex,chub/coreos-dex,ericchiang/dex,holgerkoser/dex,chub/coreos-dex,cjyar/dex,wolfeidau/dex,ericchiang/dex,coreos/dex,cjyar/dex,jwmaag/dex,circleci/dex