Auto merge of #5447 - ehuss:ws-error-message, r=alexcrichton
authorbors <bors@rust-lang.org>
Tue, 1 May 2018 13:50:13 +0000 (13:50 +0000)
committerbors <bors@rust-lang.org>
Tue, 1 May 2018 13:50:13 +0000 (13:50 +0000)
Better error message for `cargo rustc` in a workspace.

Fixes #5371


Trivial merge