Fixed: Report certificate validation failures when configuring Plex Media Server connection

Closes #2520

(cherry picked from commit ec62884649f7af5f0a29346741754590e6de99ce)

src/NzbDrone.Core/Notifications/Plex/Server/PlexServerProxy.cs

@@ -195,7 +195,12 @@ namespace NzbDrone.Core.Notifications.Plex.Server
             }
             catch (WebException ex)
             {
-                throw new PlexException("Unable to connect to Plex Media Server", ex);
+                if (ex.Status == WebExceptionStatus.TrustFailure)
+                {
+                    throw new PlexException("Unable to connect to Plex Media Server, certificate validation failed.", ex);
+                }
+
+                throw new PlexException($"Unable to connect to Plex Media Server, {ex.Message}", ex);
             }
 
             return response.Content;

src/NzbDrone.Core/Notifications/Plex/Server/PlexServerService.cs

@@ -8,6 +8,7 @@ using NLog;
 using NzbDrone.Common.Cache;
 using NzbDrone.Common.Extensions;
 using NzbDrone.Core.Music;
+using NzbDrone.Core.Validation;
 
 namespace NzbDrone.Core.Notifications.Plex.Server
 {
@@ -192,13 +193,21 @@ namespace NzbDrone.Core.Notifications.Plex.Server
             }
             catch (PlexAuthenticationException ex)
             {
-                _logger.Error(ex, "Unable to connect to Plex Server");
+                _logger.Error(ex, "Unable to connect to Plex Media Server");
                 return new ValidationFailure("AuthToken", "Invalid authentication token");
             }
+            catch (PlexException ex)
+            {
+                return new NzbDroneValidationFailure("Host", ex.Message);
+            }
             catch (Exception ex)
             {
-                _logger.Error(ex, "Unable to connect to Plex Server");
-                return new ValidationFailure("Host", "Unable to connect to Plex Server");
+                _logger.Error(ex, "Unable to connect to Plex Media Server");
+
+                return new NzbDroneValidationFailure("Host", "Unable to connect to Plex Media Server")
+                       {
+                           DetailedDescription = ex.Message
+                       };
             }
 
             return null;