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
Intern strips the error message off the top of the stack to keep from printing the message twice when displaying errors. It does so by checking if the error message is at the beginning of the stack. Intern could be a bit looser about its checking, but it seemed better to keep the error self-consistent.