Auto merge of #4398 - debris:url_4394, r=alexcrichton
authorbors <bors@rust-lang.org>
Fri, 11 Aug 2017 17:12:45 +0000 (17:12 +0000)
committerbors <bors@rust-lang.org>
Fri, 11 Aug 2017 17:12:45 +0000 (17:12 +0000)
Fixed panic when canonicalizing not-a-base-url, fixes #4394

Fixes #4394. Majority of the changes are caused by propagating error upstream to `SourceId` and `GitSource`.


Trivial merge