Fix AssertNoError to detect page exception

BTCPayServer.Tests/Extensions.cs

@@ -33,7 +33,8 @@ namespace BTCPayServer.Tests
                 foreach (var dangerAlert in driver.FindElements(By.ClassName("alert-danger")))
                     Assert.False(dangerAlert.Displayed, $"No alert should be displayed, but found this on {driver.Url}: {dangerAlert.Text}");
             }
-            Assert.DoesNotContain("ErrorPage", driver.PageSource);
+            Assert.DoesNotContain("errors", driver.Url);
+            Assert.DoesNotContain("Error", driver.Title, StringComparison.OrdinalIgnoreCase);
         }
 
         public static T AssertViewModel<T>(this IActionResult result)

BTCPayServer/Views/UIError/_LayoutError.cshtml

@@ -13,8 +13,7 @@
 </div>
 
 <center>
-	@*Marker for tests*@
-    <hr id="ErrorPage" class="primary" />
+    <hr class="primary" />
 </center>
 
 <p class="lead text-center">