Improve indexer health check messages (#1015)

* Improve indexer health check messages

Fixed: Improve health check message when all enabled indexers are disabled due to failures
Closes #1551

* Fixed: Health check failing and preventing others from running

* Fixed Indexer Health Checks and tests.

src/NzbDrone.Common/Extensions/XmlExtensions.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Xml.Linq;
+
+namespace NzbDrone.Common.Extensions
+{
+    public static class XmlExtensions
+    {
+        public static IEnumerable<XElement> FindDecendants(this XContainer container, string localName)
+        {
+            return container.Descendants().Where(c => c.Name.LocalName.Equals(localName, StringComparison.InvariantCultureIgnoreCase));
+        }
+    }
+}