[FIX] portal: show signing error messages returned by the server

The funny self.$('#o_portal_sign_accept') line was some leftover
debugging code. It doesn't actually match anything, because that
element is not in self.$el.

Before this change nothing happened when you clicked the button and an
error occured.

Now the `data.error` message (e.g. 'Invalid Order') is shown near the
bottom of the dialog.

addons/portal/static/src/js/portal_signature.js

@@ -80,8 +80,9 @@ odoo.define('portal.signature_form', function (require){
                 },
             }).then(function (data) {
                 self.$('.fa-spinner').remove();
-                self.$('#o_portal_sign_accept').prepend('<div>PROUT' + data + '</div>');
                 if (data.error) {
+                    self.$('.o_portal_sign_error_msg').remove();
+                    $confirm_btn.before(qweb.render('portal.portal_signature_error', {message: data.error}));
                     $confirm_btn.attr('disabled', false);
                 }
                 else if (data.success) {

addons/portal/static/src/scss/portal.scss

@@ -394,6 +394,10 @@ img.o_we_custom_image {
     }
 }
 
+.o_portal_signature_form .o_portal_sign_error_msg {
+    color: $brand-danger;
+}
+
 .label-optional {
     font-weight: normal;
 }

addons/portal/static/src/xml/portal_signature.xml

@@ -27,4 +27,9 @@
             <a t-att-href="widget.redirect_url">Click here to see your document.</a>
         </div>
     </t>
+    <t t-name="portal.portal_signature_error">
+        <div class="o_portal_sign_error_msg">
+            <t t-esc="message"/>
+        </div>
+    </t>
 </templates>