Fix empty result set error handling and metadata service returning correct type name (#535)

- HandleSimpleExecuteRequest now handles the case where no rows are in a result by cleanly returning a success message but with no rows included. This is handled in the front-end instead and goes through the standard path (with a clean explanation message) instead of showing a `error: no results to return`
- MetadataService was always meant to include the type name in the return result, as otherwise the front end has to guess. In order to fix a bug where this resulted in scripting based on the metadata failing (as front-end used `Procedure` instead of `StoredProcedure`), I'm returning the data here. I'll have a matching front end fix but this is overall a good solution to have.

src/Microsoft.SqlTools.ServiceLayer/Metadata/MetadataService.cs

@@ -184,22 +184,27 @@ namespace Microsoft.SqlTools.ServiceLayer.Metadata
                         var objectType = reader[2] as string;
 
                         MetadataType metadataType;
+                        string metadataTypeName;
                         if (objectType.StartsWith("V"))
                         {
                             metadataType = MetadataType.View;
+                            metadataTypeName = "View";
                         }
                         else if (objectType.StartsWith("P"))
                         {
                             metadataType = MetadataType.SProc;
+                            metadataTypeName = "StoredProcedure";
                         }
                         else
                         {
                             metadataType = MetadataType.Table;
+                            metadataTypeName = "Table";
                         }
 
                         metadata.Add(new ObjectMetadata
                         {
                             MetadataType = metadataType,
+                            MetadataTypeName = metadataTypeName,
                             Schema = schemaName,
                             Name = objectName
                         });