Use int for error_code in error controller
Workaround: Also pass error message through environment

Before that commit, error messages were passed through the HTTP response
using some webob hack. This however doesn't allow to add arbitrary HTML
(e.g. links).

Therefore this commit passes the error message through the WSGI
environment additionally in order to allow passing arbitrary HTML.

An alternative to that would be to use the webob mechanism and unencode
the HTML in the error controller.

I don't really like either option, but this one is rather simple, so I
prefered that.
Only show notify developers message for error 500
Better error message for CSRF alerts
Actually show fallback error messages

Strip error message extracted from the WebOb response stub in order to
actually show fallback messages.
Add 404 default error message
Also propose to inform developers for error code 400
Merge pull request #885 from nidico/feature-better-error-messages

Some error message improvements / better CSRF message