Prevent crash when stocktype has a negative sale price

Related issue: #281 on github.

We may want to support stock with a negative price in the future (for
example, where the "stock" is a limited number of discounts) but for now
let's just pop up an error when someone attempts to sell stock that has
a negative price.

quicktill/register.py

@@ -1347,6 +1347,14 @@ def _sell_stocktype(self, stocktype, mod):
         if explicitprice:
             sale.price = explicitprice
 
+        if sale.price < zero:
+            log.info("_sell_stocktype: negative price for %s", st.format())
+            ui.infopopup(
+                [f"{st} has a negative sale price, which is not "
+                 f"currently supported."],
+                title=f"{st} has negative price")
+            return
+
         total_qty = items * sale.qty
         sell, unallocated, remaining = st.calculate_sale(total_qty)
 
@@ -1535,6 +1543,15 @@ def _sell_stockline(self, stockline, mod):
         if explicitprice:
             sale.price = explicitprice
 
+        if sale.price < zero:
+            log.info("_sell_stockline: negative price for %s",
+                     sale.stocktype.format())
+            ui.infopopup(
+                [f"{sale.stocktype} has a negative sale price, which is not "
+                 f"currently supported."],
+                title=f"{sale.stocktype} has negative price")
+            return
+
         total_qty = items * sale.qty
         sell, unallocated, remaining = stockline.calculate_sale(
             total_qty)