Auto merge of #3593 - Susurrus:master, r=alexcrichton

Improve error for dependencies that don't have the same source paths

I've added an additional test case which is how my project compiled. Part of the issue was that I didn't know that each dependency needs to use the same path for all build targets. The previous error message was unclear both in what was going on and how to resolve it. The new error message should be more clear.

This is tied to #3592.