Replaced the error message for invalid use of extension Add method in expression lambda.

src/Compilers/CSharp/Portable/CSharpResources.Designer.cs

@@ -3923,7 +3923,7 @@ internal class CSharpResources {
         }
         
         /// <summary>
-        ///   Looks up a localized string similar to An expression tree lambda may not contain an extension collection element initializer..
+        ///   Looks up a localized string similar to An extension Add method is not supported for a collection initializer in an expression lambda..
         /// </summary>
         internal static string ERR_ExtensionCollectionElementInitializerInExpressionTree {
             get {

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -4545,7 +4545,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
     <value>An expression tree lambda may not contain a dictionary initializer.</value>
   </data>
   <data name="ERR_ExtensionCollectionElementInitializerInExpressionTree" xml:space="preserve">
-    <value>An expression tree lambda may not contain an extension collection element initializer.</value>
+    <value>An extension Add method is not supported for a collection initializer in an expression lambda.</value>
   </data>
   <data name="IDS_FeatureNameof" xml:space="preserve">
     <value>nameof operator</value>