Determine if a supplier supplies all products in a list

Assuming that idProduct, idSupplier are unique*, here is a simple query:

SELECT 1
FROM SupplierCatalog
WHERE idProduct IN (10, 26, 40, 42, 50) AND idSupplier = 5
HAVING COUNT(*) = 5 -- this is the number of items in the above list

This returns 0 or 1 rows which is all you need to check in your code.

_{* If duplicates are allowed/present change to HAVING COUNT(DISTINCT idProduct) ...}

SELECT idproduct 
  FROM SupplierCatalog 
 WHERE idProduct IN (10, 26, 40, 42, 50) 
   AND idSupplier = 5 
 GROUP BY idproduct 
HAVING COUNT(*) = 5

0 rows is false and 1 row is true

You can remove the AND idSupplier = 5 and get all the suppliers