Improve PayButton error page (#4129)

As this is a public page we should embed it in the non-navigation layout. Also improves the error display.

BTCPayServer/Views/UIPublic/PayButtonHandle.cshtml

@@ -1,17 +1,14 @@
-@{ 
+@{
+    Layout = "_LayoutSimple";
     var allErrors = ViewData.ModelState.Values.SelectMany(v => v.Errors.Select(b => b.ErrorMessage));
 }
 
-
 <div class="row">
     <div class="col-lg-12">
         <h2>Pay Button request failed</h2>
-        Please fix following errors:
-        <ul>
-            @foreach (var error in allErrors)
-            {
-                <li>@error</li>
-            }
-        </ul>
+        @foreach (var error in allErrors)
+        {
+            <div class="alert alert-danger mt-4">@error</div>
+        }
     </div>
 </div>