Add Ex prefix to error message id

Author: SergeiPavlov

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

@@ -133,7 +133,7 @@ protected override Expression VisitQueryableMethod(MethodCallExpression mc, Quer
 
             break;
           case QueryableMethodKind.DistinctBy:
-            throw new NotSupportedException(Strings.UnsupportedDistinctBy);
+            throw new NotSupportedException(Strings.ExUnsupportedDistinctBy);
           case QueryableMethodKind.First:
           case QueryableMethodKind.FirstOrDefault:
           case QueryableMethodKind.Single:

Orm/Xtensive.Orm/Strings.Designer.cs

@@ -2573,7 +2573,7 @@ Error: {1}</value>
   <data name="ExTagDoesNotSupportQueryProviderOfTypeX" xml:space="preserve">
     <value>'Tag' does not support query provider of type '{0}'.</value>
   </data>
-  <data name="UnsupportedDistinctBy" xml:space="preserve">
+  <data name="ExUnsupportedDistinctBy" xml:space="preserve">
     <value>IQueryable.DistinctBy() extension is not supported. Use '.AsEnumerable().DistinctBy()' instead.</value>
   </data>
 </root>