auto merge of #776 : alexcrichton/cargo/issue-488, r=alexcrichton
authorbors <bors@rust-lang.org>
Wed, 5 Nov 2014 18:08:33 +0000 (18:08 +0000)
committerbors <bors@rust-lang.org>
Wed, 5 Nov 2014 18:08:33 +0000 (18:08 +0000)
This helps us not unwrap() and rather propagate the error upwards.

Closes #488


Trivial merge