Return more informative errors from request/node

This makes the messages emitted by request/node on connection failure more informative.

Fixes https://github.com/theintern/leadfoot/issues/12
Yeah, good call. I thought that @ had to be escaped outside of the authority-part but it turns out that the URI grammar defines @ as one of the characters that can exist unescaped within the path-part of a URI. Keep this this way, and get rid of the regular expression in Leadfoot to ensure strict compliance.