Return a StatusCodeError when a workspace's message limit is exceeded (#1383)

Currently, this library throws a `strconv.NumError` when attempting to
post messages to a Slack workspace that has exceeded its message limit.
It does so because Slack returns a 429 without a `Retry-After` header
for those requests.

This PR improves the situation by instead returning a `StatusCodeError`.
I couldn't see any obvious way to provide a more detailed error without
creating a new error class. To only additional information returned from
Slack for these errors is a body that reads `message_limit_exceeded`.

Author: nlopes

misc.go

@@ -306,7 +306,7 @@ func timerReset(t *time.Timer, d time.Duration) {
 }
 
 func checkStatusCode(resp *http.Response, d Debug) error {
-	if resp.StatusCode == http.StatusTooManyRequests {
+	if resp.StatusCode == http.StatusTooManyRequests && resp.Header.Get("Retry-After") != "" {
 		retry, err := strconv.ParseInt(resp.Header.Get("Retry-After"), 10, 64)
 		if err != nil {
 			return err

webhooks_test.go

@@ -66,6 +66,25 @@ func TestPostWebhook_NotOK(t *testing.T) {
 	}
 }
 
+func TestPostWebhook_MessageLimitExceeded(t *testing.T) {
+	once.Do(startServer)
+
+	http.HandleFunc("/message_limit_exceeded", func(rw http.ResponseWriter, r *http.Request) {
+		// When a workspace's message limit is exceeded we get a 429 without a Retry-After header
+		rw.WriteHeader(http.StatusTooManyRequests)
+		rw.Write([]byte("message_limit_exceeded"))
+	})
+
+	url := "http://" + serverAddr + "/message_limit_exceeded"
+
+	err := PostWebhook(url, &WebhookMessage{})
+
+	if err == nil {
+		t.Errorf("Expected to receive error")
+	}
+	assert.IsType(t, StatusCodeError{}, err)
+}
+
 func TestWebhookMessage_WithBlocks(t *testing.T) {
 	textBlockObject := NewTextBlockObject("plain_text", "text", false, false)
 	sectionBlock := NewSectionBlock(textBlockObject, nil, nil)