couple improvements :)

First of all the commit 2237167 which github shows as part of the pull request is complete `unknown` to me. But I guess it can be quickly resolved and I can easily blame `ungit` (https://github.com/FredrikNoren/ungit) as I've used it to do the git related tasks.

anyway in order of appearance: 
* render erroface page only if req accepts html, as per general errors from api routes can and will work with json, and most probably will have their own handler for error handling, so I guess that is not entirely needed. If you object, just exclude it :)
* do not throw error on missing or empty error line
* parse headers improvements. Namely `jade` support. Before them errorface just threw exception on errors produced by invalid jade syntax .