Auto merge of #4492 - RalfJung:virtual, r=matklad
authorbors <bors@rust-lang.org>
Thu, 14 Sep 2017 20:12:16 +0000 (20:12 +0000)
committerbors <bors@rust-lang.org>
Thu, 14 Sep 2017 20:12:16 +0000 (20:12 +0000)
Fix confusing error and docs wrt. virtual manifests

Fixes #3966

Contains updated version of #3967


Trivial merge