Added URL to context resolution error message.

[davidlehn]

• It's unfortunate to duplicate like this, but perhaps helpful. In theory the details and cause should still be looked at to possibly find a lower level issue. I'm not sure how to cleanly indicate this.
• Would it be better to put the URL at the end? Would make the common text easier to read before the potentially long variable content.
• The node doc loader outputs something similar for regular invalid URLs. It doesn't handle garbage URLs very well and puts them directly in the error message without good sanitizing. I think that problem will happen before URLs get here.