[hack] Add more info to isset/empty error
[hack] Add more info to isset/empty error

Summary: As someone who doesn't write a lot of Hack, it wasn't obvious what I should use instead.

Reviewed By: savil

Differential Revision: D4308817

fbshipit-source-id: 38ea7c957572504a46d4fc4f9093ccfe8496b523

ForbesLindesay/flow,gabelevi/flow,gabro/flow,ForbesLindesay/flow,nmote/flow,gabro/flow,jamesgpearce/flow,samwgoldman/flow,mroch/flow,claudiopro/flow,JonathanUsername/flow,popham/flow,ylu1317/flow,samwgoldman/flow,AgentME/flow,popham/flow,ForbesLindesay/flow,MichaelDeBoey/flow,fletcherw/flow,JonathanUsername/flow,MichaelDeBoey/flow,samwgoldman/flow,mroch/flow,nmote/flow,ForbesLindesay/flow,fletcherw/flow,nmote/flow,fletcherw/flow,facebook/flow,nmote/flow,claudiopro/flow,fletcherw/flow,JonathanUsername/flow,gabelevi/flow,TiddoLangerak/flow,gabro/flow,gabro/flow,AgentME/flow,ylu1317/flow,JonathanUsername/flow,mroch/flow,mroch/flow,MichaelDeBoey/flow,TiddoLangerak/flow,JonathanUsername/flow,jamesgpearce/flow,mroch/flow,ylu1317/flow,jamesgpearce/flow,nmote/flow,popham/flow,samwgoldman/flow,popham/flow,gabelevi/flow,popham/flow,JonathanUsername/flow,facebook/flow,TiddoLangerak/flow,facebook/flow,claudiopro/flow,mroch/flow,MichaelDeBoey/flow,jamesgpearce/flow,ylu1317/flow,facebook/flow,ylu1317/flow,ForbesLindesay/flow,JonathanUsername/flow,facebook/flow,fletcherw/flow,gabelevi/flow,jamesgpearce/flow,TiddoLangerak/flow,MichaelDeBoey/flow,jamesgpearce/flow,AgentME/flow,fletcherw/flow,nmote/flow,TiddoLangerak/flow,jamesgpearce/flow,samwgoldman/flow,samwgoldman/flow,gabro/flow,ForbesLindesay/flow,gabelevi/flow,gabro/flow,facebook/flow,AgentME/flow,mroch/flow,claudiopro/flow,nmote/flow,samwgoldman/flow,popham/flow,AgentME/flow,gabelevi/flow,facebook/flow,claudiopro/flow,popham/flow,gabelevi/flow