New: Don't treat 400 responses from Notifiarr as errors

(cherry picked from commit 5eb420bbe12f59d0a5392abf3d351be28ca210e6)

Closes #4137

src/NzbDrone.Core/Notifications/Notifiarr/NotifiarrProxy.cs

@@ -53,7 +53,10 @@ namespace NzbDrone.Core.Notifications.Notifiarr
                         _logger.Error("HTTP 401 - API key is invalid");
                         throw new NotifiarrException("API key is invalid");
                     case 400:
-                        throw new NotifiarrException("Unable to send notification. Ensure Lidarr Integration is enabled & assigned a channel on Notifiarr");
+                        // 400 responses shouldn't be treated as an actual error because it's a misconfiguration
+                        // between Lidarr and Notifiarr for a specific event, but shouldn't stop all events.
+                        _logger.Error("HTTP 400 - Unable to send notification. Ensure Lidarr Integration is enabled & assigned a channel on Notifiarr");
+                        break;
                     case 502:
                     case 503:
                     case 504: