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.
from cubicweb.cwconfig import CubicWebConfiguration
def includeme(config):
appid = config.registry.settings['cubicweb.instance']
cwconfig = CubicWebConfiguration.config_for(appid)
config.registry['cubicweb.config'] = cwconfig
config.registry['cubicweb.repository'] = repo = cwconfig.repository()
config.registry['cubicweb.registry'] = repo.vreg