Auto merge of #5250 - rleungx:tweak-error-message, r=matklad
authorbors <bors@rust-lang.org>
Tue, 27 Mar 2018 13:16:48 +0000 (13:16 +0000)
committerbors <bors@rust-lang.org>
Tue, 27 Mar 2018 13:16:48 +0000 (13:16 +0000)
tweak error message

Closes #5238.


Trivial merge