[FIX] account: fix unreconciling invoices, raising a wrong error.

That bug was introduced when removing the invoice workflow, where a call to action_invoice_cancel() has been used instead of action_invoice_reopen() which does slightly different things, as you can imagine.

addons/account/models/account_invoice.py

@@ -353,7 +353,7 @@ class AccountInvoice(models.Model):
         reconciled = self.filtered(lambda invoice: invoice.reconciled)
         not_reconciled = self - reconciled
         (reconciled & pre_reconciled).filtered(lambda invoice: invoice.state == 'open').action_invoice_paid()
-        (not_reconciled & pre_not_reconciled).filtered(lambda invoice: invoice.state == 'paid').action_invoice_cancel()
+        (not_reconciled & pre_not_reconciled).filtered(lambda invoice: invoice.state == 'paid').action_invoice_re_open()
         return res
 
     @api.model
@@ -578,7 +578,7 @@ class AccountInvoice(models.Model):
     @api.multi
     def action_invoice_cancel(self):
         if self.filtered(lambda inv: inv.state not in ['proforma2', 'draft', 'open']):
-            raise UserError(_("Invoice must be in draft or Pro-forma state in order to validate it."))
+            raise UserError(_("Invoice must be in draft,Pro-forma or open state in order to be cancelled."))
         return self.action_cancel()
 
     @api.multi