Add validation when "declaring credit" for receiving batch row

i.e. don't just blindly attempt, when it isn't supported

tailbone/templates/receiving/declare_credit.mako

@@ -39,6 +39,22 @@
 <div style="display: flex; justify-content: space-between;">
 
   <div class="form-wrapper">
+
+    <p style="padding: 1em;">
+      Please select the "state" of the product, and enter the appropriate
+      quantity.
+    </p>
+
+    <p style="padding: 1em;">
+      Note that this tool will <strong>deduct</strong> from the "received"
+      quantity, and <strong>add</strong> to the corresponding credit quantity.
+    </p>
+
+    <p style="padding: 1em;">
+      Please see ${h.link_to("Receive Row", url('{}.receive_row'.format(route_prefix), uuid=batch.uuid, row_uuid=row.uuid))}
+      if you need to "receive" instead of "convert" the product.
+    </p>
+
     ${form.render()|n}
   </div><!-- form-wrapper -->
 

tailbone/templates/receiving/receive_row.mako

@@ -39,6 +39,22 @@
 <div style="display: flex; justify-content: space-between;">
 
   <div class="form-wrapper">
+
+    <p style="padding: 1em;">
+      Please select the "state" of the product, and enter the appropriate
+      quantity.
+    </p>
+
+    <p style="padding: 1em;">
+      Note that this tool will <strong>add</strong> the corresponding
+      quantities for the row.
+    </p>
+
+    <p style="padding: 1em;">
+      Please see ${h.link_to("Declare Credit", url('{}.declare_credit'.format(route_prefix), uuid=batch.uuid, row_uuid=row.uuid))}
+      if you need to "convert" some already-received amount, into a credit.
+    </p>
+
     ${form.render()|n}
   </div><!-- form-wrapper -->
 

tailbone/views/purchasing/receiving.py

@@ -861,6 +861,7 @@ class ReceivingBatchView(PurchasingBatchView):
 
         schema = ReceiveRowForm().bind(session=self.Session())
         form = forms.Form(schema=schema, request=self.request)
+        form.cancel_url = self.get_row_action_url('view', row, mobile=mobile)
         form.set_widget('mode', forms.widgets.JQuerySelectWidget(values=[(m, m) for m in possible_modes]))
         form.set_widget('quantity', forms.widgets.CasesUnitsWidget(amount_required=True,
                                                                    one_amount_only=True))
@@ -996,7 +997,7 @@ class ReceivingBatchView(PurchasingBatchView):
             'allow_cases': self.handler.allow_cases(),
         }
 
-        schema = DeclareCreditForm().bind(session=self.Session())
+        schema = DeclareCreditForm()
         form = forms.Form(schema=schema, request=self.request)
         form.set_widget('credit_type', forms.widgets.JQuerySelectWidget(
             values=[(m, m) for m in possible_credit_types]))
@@ -1011,9 +1012,16 @@ class ReceivingBatchView(PurchasingBatchView):
             kwargs['cases'] = kwargs['quantity']['cases']
             kwargs['units'] = kwargs['quantity']['units']
             del kwargs['quantity']
-            self.handler.declare_credit(row, **kwargs)
+            try:
+                result = self.handler.can_declare_credit(row, **kwargs)
+            except Exception as error:
+                self.request.session.flash("Handler says you can't declare that credit: {}".format(error), 'error')
+            else:
+                if result:
+                    self.handler.declare_credit(row, **kwargs)
+                    return self.redirect(self.get_row_action_url('view', row))
 
-            return self.redirect(self.get_row_action_url('view', row))
+                self.request.session.flash("Handler says you can't declare that credit; not sure why", 'error')
 
         context['form'] = form
         context['dform'] = form.make_deform_form()