OCC-423: Fix broken tax rate 0 case and improve exception text. (#708)

* Add better exception text.

* Fix broken tax rate 0 case.

Author: sarahelsaig

src/Modules/OrchardCore.Commerce/Abstractions/ITaxProvider.cs

@@ -25,7 +25,10 @@ protected static async Task<bool> AllOrNoneAsync(
 
         if (count == 0) return false;
         if (count == items.Count) return true;
-        throw new InvalidOperationException("Some, but not all products have gross price. This is invalid.");
+        throw new InvalidOperationException(
+            "Some, but not all products have gross price. This is invalid. If you want to declare a tax-free " +
+            "product, make sure to still include a Tax Part for the content type. Then set the Tax Rate field to 0 " +
+            "in the products.");
     }
 
     /// <inheritdoc cref="AllOrNoneAsync(PromotionAndTaxProviderContext, Func{IList{PromotionAndTaxProviderContextLineItem}, Task{int}})"/>

src/Modules/OrchardCore.Commerce/Services/LocalTaxProvider.cs

@@ -39,6 +39,6 @@ public Task<PromotionAndTaxProviderContext> UpdateAsync(PromotionAndTaxProviderC
 
     public Task<bool> IsApplicableAsync(PromotionAndTaxProviderContext model) =>
         ITaxProvider.AllOrNoneAsync(model, items => items
-            .Select(item => item.Content.ContentItem.As<TaxPart>())
-            .Count(taxPart => taxPart?.GrossPrice?.Amount.IsValid == true && taxPart.TaxRate.Value >= 0));
+            .SelectWhere(item => item.Content.ContentItem.As<TaxPart>())
+            .Count(taxPart => taxPart.TaxRate.Value == 0 || (taxPart.GrossPrice.Amount.IsValid && taxPart.TaxRate.Value > 0)));
 }