fix(api): better panic recovery in SSE (#3517)

to handle:
```
api       | 2018-10-29 10:30:15 | err  | [PANIC_RECOVERY] Panic occurred on GET:/events?lastEventId=&r=5076636469124916, recover index > windowEnd
api       | 2018-10-29 10:30:15 | err  | [PANIC_RECOVERY] Stacktrace of 4096 bytes
```

engine/api/events.go

@@ -157,13 +157,19 @@ func (b *eventsBroker) Start(ctx context.Context) {
 }
 
 func (b *eventsBroker) ServeHTTP() service.Handler {
-	return func(ctx context.Context, w http.ResponseWriter, r *http.Request) error {
+	return func(ctx context.Context, w http.ResponseWriter, r *http.Request) (err error) {
 		// Make sure that the writer supports flushing.
 		f, ok := w.(http.Flusher)
 		if !ok {
 			return sdk.WrapError(fmt.Errorf("streaming unsupported"), "")
 		}
 
+		defer func() {
+			if r := recover(); r != nil {
+				err = fmt.Errorf("eventsBroker.ServeHTTP recovered %v", r)
+			}
+		}()
+
 		user := getUser(ctx)
 		if err := loadUserPermissions(b.dbFunc(), b.cache, user); err != nil {
 			return sdk.WrapError(err, "eventsBroker.Serve Cannot load user permission")
@@ -270,6 +276,12 @@ func (client *eventsBrokerSubscribe) Send(event sdk.Event) (err error) {
 	client.mutex.Lock()
 	defer client.mutex.Unlock()
 
+	defer func() {
+		if r := recover(); r != nil {
+			err = fmt.Errorf("eventsBrokerSubscribe.Send recovered %v", r)
+		}
+	}()
+
 	if client == nil || client.w == nil {
 		return nil
 	}
@@ -298,12 +310,6 @@ func (client *eventsBrokerSubscribe) Send(event sdk.Event) (err error) {
 		return nil
 	}
 
-	defer func() {
-		if r := recover(); r != nil {
-			err = fmt.Errorf("%v", r)
-		}
-	}()
-
 	if _, err := client.w.Write(buffer.Bytes()); err != nil {
 		return sdk.WrapError(err, "unable to write to client")
 	}