SQL : stored procedure won't return any error message or number

Here is my insert stored procedure:

CREATE PROCEDURE spainsert 
    @A_Name nvarchar(50) 
AS BEGIN 
   BEGIN TRY
      DECLARE Detailed_Error VAR_CHAR(MAX); 

      Insert into A(A_Name) 
      values(A_Name) 
   END TRY 
   BEGIN CATCH
       SELECT 
          Detailed_Error = CAST(ERROR_NUMBER() AS VAR_CHAR ) + ' : '+ ERROR_MESSAGE() + ' : ' + CAST(ERROR_LINE() AS VAR_CHAR );
   END CATCH 
END 

When I try to enter NUMBER instead of NAME : exec spa insert 500, it won't show any error, but just saves that row into table. Could you please help me how to handle errors, how to know the number and the error message ?!

Thank you all ! 123456787523535464654645654651541541255435.4153241653241636524165324135