FieldRetriever.GetFieldId: add param to indicate if missing field needs logging


The FieldRetriever now has an extra overload method for GetFieldId: GetFieldId(string label, bool warnWhenNotFound). The newly introduced boolean parameter warnWhenNotFound can be used to disable the logging in the error-file in case the field doesn't exist. The logging is by default enabled in case of the method GetFieldId(string label).

The Fieldhandler's method HasField() will no longer log an error in case the FieldHandler doesn't have the field.