Extend webhook validation error handling

- Move webhook URL prefixes to constants list
  - export so that client code may reference them
    with any validation work that they perform

- Collect and return user-provided webhook URL alongside
  required URL prefixes to help aid with troubleshooting
  the cause of any errors that they may receive

refs #6 (loosely)

send.go

@@ -11,6 +11,12 @@ import (
 	"time"
 )
 
+// Known webhook URL prefixes for submitting messages to Microsoft Teams
+const (
+	WebhookURLOfficecomPrefix = "https://outlook.office.com"
+	WebhookURLOffice365Prefix = "https://outlook.office365.com"
+)
+
 // API - interface of MS Teams notify
 type API interface {
 	Send(webhookURL string, webhookMessage MessageCard) error
@@ -80,19 +86,29 @@ func IsValidInput(webhookMessage MessageCard, webhookURL string) (bool, error) {
 // IsValidWebhookURL performs validation checks on the webhook URL used to
 // submit messages to Microsoft Teams.
 func IsValidWebhookURL(webhookURL string) (bool, error) {
-	// basic URL check
-	_, err := url.Parse(webhookURL)
-	if err != nil {
-		return false, err
-	}
-	// only pass MS teams webhook URLs
+
 	switch {
-	case strings.HasPrefix(webhookURL, "https://outlook.office.com/webhook/"):
-	case strings.HasPrefix(webhookURL, "https://outlook.office365.com/webhook/"):
+	case strings.HasPrefix(webhookURL, WebhookURLOfficecomPrefix):
+	case strings.HasPrefix(webhookURL, WebhookURLOffice365Prefix):
 	default:
-		err = errors.New("invalid ms teams webhook url")
-		return false, err
+		u, err := url.Parse(webhookURL)
+		if err != nil {
+			return false, fmt.Errorf(
+				"unable to parse webhook URL %q: %v",
+				webhookURL,
+				err,
+			)
+		}
+		userProvidedWebhookURLPrefix := u.Scheme + "://" + u.Host
+
+		return false, fmt.Errorf(
+			"webhook URL does not contain expected prefix; got %q, expected one of %q or %q",
+			userProvidedWebhookURLPrefix,
+			WebhookURLOfficecomPrefix,
+			WebhookURLOffice365Prefix,
+		)
 	}
+
 	return true, nil
 }