Throw a helpful message when users run "arc diff --raw --update".
Throw a helpful message when users run "arc diff --raw --update".

Summary: We crash and burn right now, trying to launch an interactive editor against a non-tty stdin. Raise a helpful error message instead.

Test Plan: Will run "arc diff --raw --update".

Reviewers: btrahan, yairlivne

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1949

memsql/arcanist,kevinseelbach/arcanist,aik099/arcanist,uber/arcanist,phacility/arcanist,phacility/arcanist,PageUpPeopleOrg/arcanist-patches,aik099/arcanist,hach-que/arcanist,calblueprint/arcanist,wikimedia/phabricator-arcanist,calblueprint/arcanist,hach-que/arcanist,jra3/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,Symplicity/arcanist,alexmv/arcanist,kevinseelbach/arcanist,memsql/arcanist,r4nt/arcanist,fivestars/arcanist,wharris/arcanist,uber/arcanist,sneakpeeq/arcanist,aik099/arcanist,codelabs-ch/arcanist,wikimedia/phabricator-arcanist,Khan/arcanist,jra3/arcanist,dguo-coursera/arcanist,wikimedia/phabricator-arcanist,phacility/arcanist,hach-que/arcanist,calblueprint/arcanist,hach-que/arcanist,r4nt/arcanist,phacility/arcanist,jra3/arcanist,sneakpeeq/arcanist,phacility/arcanist,folsom-labs/arcanist,KieranHayes/arcanist,wharris/arcanist,dguo-coursera/arcanist,folsom-labs/arcanist,Symplicity/arcanist,eSpark/arcanist,Khan/arcanist,wikimedia/phabricator-arcanist,KieranHayes/arcanist,KieranHayes/arcanist,uber/arcanist,sneakpeeq/arcanist,dguo-coursera/arcanist,haskell-infra/arcanist,eSpark/arcanist,r4nt/arcanist,zhihu/arcanist,memsql/arcanist,alexmv/arcanist,codelabs-ch/arcanist,julkiewicz/arcanist,folsom-labs/arcanist,PageUpPeopleOrg/arcanist-patches,eSpark/arcanist,chundongwang/arcanist,Symplicity/arcanist,chundongwang/arcanist,zhihu/arcanist,calblueprint/arcanist,julkiewicz/arcanist,Khan/arcanist,folsom-labs/arcanist,wharris/arcanist,wxstars/arcanist,r4nt/arcanist,dguo-coursera/arcanist,Khan/arcanist,PageUpPeopleOrg/arcanist-patches,sneakpeeq/arcanist,Symplicity/arcanist,julkiewicz/arcanist,coursera/arcanist,coursera/arcanist,kevinseelbach/arcanist,julkiewicz/arcanist,fivestars/arcanist,chundongwang/arcanist,wharris/arcanist,KieranHayes/arcanist,zhihu/arcanist,coursera/arcanist,haskell-infra/arcanist,zhihu/arcanist,aik099/arcanist,haskell-infra/arcanist,coursera/arcanist,eSpark/arcanist,chundongwang/arcanist,uber/arcanist,alexmv/arcanist,fivestars/arcanist,wxstars/arcanist,fivestars/arcanist,jra3/arcanist,alexmv/arcanist,wxstars/arcanist,wxstars/arcanist,PageUpPeopleOrg/arcanist-patches,memsql/arcanist,haskell-infra/arcanist,kevinseelbach/arcanist