Improve LFW --lfwPairs error message.
Improve LFW --lfwPairs error message.

francisleunggie/openface,Alexx-G/openface,cmusatyalab/openface,nmabhi/Webface,francisleunggie/openface,nmabhi/Webface,xinfang/face-recognize,xinfang/face-recognize,Alexx-G/openface,Alexx-G/openface,cmusatyalab/openface,nmabhi/Webface,xinfang/face-recognize,nmabhi/Webface,Alexx-G/openface,francisleunggie/openface,cmusatyalab/openface