wsgi: clearer exception when CW_INSTANCE is missing instance_name is mandatory for cwcfg.config_for() Raise with a KeyError: 'CW_INSTANCE' instead of hard to read exception.

version=3
opts=uversionmangle=s/(rc|a|b|c)/~$1/ \
http://pypi.debian.net/pyramid-cubicweb/pyramid-cubicweb-(.+)\.(?:zip|tgz|tbz|txz|(?:tar\.(?:gz|bz2|xz)))