[FIX] (website_)sale_coupon: Display clearer promo code error on cart

Before this commit:
Coupon error on cart would always show "Invalid or Expired coupon code"
instead of a more accurate error (eg: already applied, can't use 2 code..).
Plus it would still show the error on page refresh because of the error being
stored in URL param.

Now:
It will show the exact error and since the error is not stored in URL, it won't
be still displayed on page refresh.

This closes #1601

controllers/main.py

@@ -10,8 +10,10 @@ class WebsiteSale(WebsiteSale):
     def pricelist(self, promo, **post):
         order = request.website.sale_get_order()
         coupon_status = request.env['sale.coupon.apply.code'].sudo().apply_coupon(order, promo)
-        if coupon_status.get('error', False):
+        if coupon_status.get('not_found'):
             return super(WebsiteSale, self).pricelist(promo, **post)
+        elif coupon_status.get('error'):
+            request.session['error_promo_code'] = coupon_status['error']
         return request.redirect(post.get('r', '/shop/cart'))
 
     @http.route(['/shop/payment'], type='http', auth="public", website=True)

models/sale_order.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 from odoo import models, fields, api
+from odoo.http import request
 
 
 class SaleOrder(models.Model):
@@ -10,3 +11,9 @@ class SaleOrder(models.Model):
         for order in self:
             reward_lines = order.website_order_line.filtered(lambda line: line.is_reward_line)
             order.cart_quantity -= int(sum(reward_lines.mapped('product_uom_qty')))
+
+    def get_promo_code_error(self, delete=True):
+        error = request.session.get('error_promo_code')
+        if error and delete:
+            request.session.pop('error_promo_code')
+        return error

views/website_sale_templates.xml

@@ -47,6 +47,13 @@
                 </div>
             </div>
         </t>
+        <t t-if="website_sale_order.get_promo_code_error(delete=False)">
+            <div class="panel panel-danger mt16">
+                <div class="panel-heading clearfix">
+                    <span class="pull-left"><t t-esc="website_sale_order.get_promo_code_error()" /></span>
+                </div>
+            </div>
+        </t>
     </xpath>
     <xpath expr="//t[@name='code_not_available']" position="replace"/>
 </template>