[BuildSystem] Error if there is no root document.
[BuildSystem] Error if there is no root document.

 - <rdar://problem/32179388> Crash with no root document

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild