cubicweb #590146 remove critical warning at startup when goa cannot find constraint [resolved]

When starting CW on a machine without logilab.constraint but with a gecode enabled RQL, you get a CRITICAL log message about goa not being able to import constraint.

The log level must be downgraded to a WARNING at most. And ideally, CW should not even try to import the goa related parts when not running on app engine.

done in3.6.0
load left0.000
closed by<not specified>