Allow (?) custom schemes on vanity imports

[sdboyer]

It should be possible to specify a custom scheme (http vs. https, possibly more) when relying on a vanity import source. But the deduction system poops out earlier than it should. Specifically, this check is wrong because it's actually failing on the wrong kind of error - we should be getting this error back, but we never actually make it that far.

[sdboyer]

At least...it seems like specifying such a scheme should be possible. Maybe it shouldn't be. That's something we may want to figure out here, too 😄

[fabulous-gopher]

This issue was moved to golang/dep#414