Remote API error handling: expose response string

Extend error handling by retrieving and providing the
response string from the remote API as part of the error
message returned to the caller if the status code
indicates an error condition.

Based on testing, the response string contains useful
troubleshooting information which can help identify
missing information in the submitted MessageCard payload.

---

Note: This commit contains example `logger` object calls
to illustrate the proposed changes mentioned in #10. Before
the associated PR would be merged, those `logger` calls
would be removed and implemented as part of a separate
PR specific to #10.

refs #10, #6 (loosely)

send.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io/ioutil"
 	"net/http"
 	"net/url"
 	"strings"
@@ -32,6 +33,9 @@ func NewClient() API {
 
 // Send - will post a notification to MS Teams webhook URL
 func (c teamsClient) Send(webhookURL string, webhookMessage MessageCard) error {
+
+	// logger.Printf("Send: Webhook message received: %#v\n", webhookMessage)
+
 	// Validate input data
 	if valid, err := IsValidInput(webhookMessage, webhookURL); !valid {
 		return err
@@ -41,20 +45,54 @@ func (c teamsClient) Send(webhookURL string, webhookMessage MessageCard) error {
 	webhookMessageByte, _ := json.Marshal(webhookMessage)
 	webhookMessageBuffer := bytes.NewBuffer(webhookMessageByte)
 
+	// JSON data before formatting changes
+	//logger.Printf("Send: %+v\n", string(webhookMessageByte))
+
+	var prettyJSON bytes.Buffer
+	if err := json.Indent(&prettyJSON, webhookMessageByte, "", "\t"); err != nil {
+		return err
+	}
+	// logger.Printf("Send: Payload for Microsoft Teams: \n\n%v\n\n", prettyJSON.String())
+
 	// prepare request (error not possible)
 	req, _ := http.NewRequest(http.MethodPost, webhookURL, webhookMessageBuffer)
 	req.Header.Add("Content-Type", "application/json;charset=utf-8")
 
 	// do the request
 	res, err := c.httpClient.Do(req)
 	if err != nil {
+		// logger.Println(err)
 		return err
 	}
+
+	// Make sure that we close the response body once we're done with it
+	defer res.Body.Close()
+
+	// Get the response body, then convert to string for use with extended
+	// error messages
+	responseData, err := ioutil.ReadAll(res.Body)
+	if err != nil {
+		// logger.Println(err)
+		return err
+	}
+
 	if res.StatusCode >= 299 {
-		err = errors.New("error on notification: " + res.Status)
+
+		// 400 Bad Response is likely an indicator that we failed to provide a
+		// required field in our JSON payload. For example, when leaving out
+		// the top level MessageCard Summary or Text field, the remote API
+		// returns "Summary or Text is required." as a text string. We include
+		// that response text in the error message that we return to the
+		// caller.
+
+		err = fmt.Errorf("error on notification: %v, %q", res.Status, string(responseData))
+		// logger.Println(err)
 		return err
 	}
 
+	// log the response string
+	// logger.Printf("Send: Response string from Microsoft Teams API: %v\n", string(responseData))
+
 	return nil
 }