Replace const string with resouse

Author: alex-kulakov

Orm/Xtensive.Orm/Orm/Linq/Translator.Queryable.cs

@@ -1175,7 +1175,7 @@ private ProjectionExpression VisitSelectMany(Expression source, LambdaExpression
                 return selectManyInfo.GroupByProjection;
               }
 
-              throw new NotImplementedException("Result selector is not supported for SelectMany over GroupBy result.");
+              throw new NotImplementedException(Strings.ExResultSelectorIsNotSupportedForSelectManyOverGrouping);
             }
           }