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.

priorityimportant
typebug
done in3.6.0
load0.100
load left0.000
closed by<not specified>