Fixed: Exception when request to SABnzbd times out

(cherry picked from commit f946d78153b85ad726a06a1140143c8beac8766d)

Closes #2518
Closes #2519

src/NzbDrone.Core/Download/Clients/Sabnzbd/SabnzbdProxy.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Net;
+using System.Net.Http;
 using Newtonsoft.Json.Linq;
 using NLog;
 using NzbDrone.Common.Extensions;
@@ -188,6 +189,10 @@ namespace NzbDrone.Core.Download.Clients.Sabnzbd
             {
                 throw new DownloadClientException("Unable to connect to SABnzbd, {0}", ex, ex.Message);
             }
+            catch (HttpRequestException ex)
+            {
+                throw new DownloadClientUnavailableException("Unable to connect to SABnzbd, {0}", ex, ex.Message);
+            }
             catch (WebException ex)
             {
                 if (ex.Status == WebExceptionStatus.TrustFailure)