Throw a human readable exception if theme resource not found by passe…

…d path in GetAssetHash method.

VirtoCommerce.LiquidThemeEngine/ShopifyLiquidThemeEngine.cs

@@ -216,6 +216,10 @@ public string GetAssetHash(string filePath)
 
                using (var stream = GetAssetStream(filePath))
                {
+                   if (stream == null)
+                   {
+                       throw new StorefrontException($"Theme resource for path '{filePath}' not found");
+                   }
                    var hashAlgorithm = CryptoConfig.AllowOnlyFipsAlgorithms ? (SHA256)new SHA256CryptoServiceProvider() : new SHA256Managed();
                    return WebEncoders.Base64UrlEncode(hashAlgorithm.ComputeHash(stream));
                }