Exception from Warp: MalformedScheme NonAlphaLeading

[pbrisbin]

This happens when you don't configure an absolute approot.

This code is called on a string like "/foo/bar" which fails because that code only works on an absolute URI.

Hat-tip to @jferris for bumping on this in the original PR. I was definitely lacking some understanding in my assumptions when I said that code was OK.

I believe an absolute approot is required for OAuth2 machinery to work anyway, but we could try to make the error message clearer.