Improve iotclient error details (#62)

Some errors are returned without an err model. In such cases, the details of the response are in the error body.

Author: Paolo Calao

Diff for: internal/iot/error.go

@@ -18,6 +18,7 @@
 package iot
 
 import (
+	"encoding/json"
 	"fmt"
 
 	iotclient "github.com/arduino/iot-client-go"
@@ -32,9 +33,17 @@ func errorDetail(err error) error {
 	}
 
 	modErr, ok := apiErr.Model().(iotclient.ModelError)
+	if ok {
+		return fmt.Errorf("%w: %s", err, modErr.Detail)
+	}
+
+	body := make(map[string]interface{})
+	if bodyErr := json.Unmarshal(apiErr.Body(), &body); bodyErr != nil {
+		return err
+	}
+	detail, ok := body["detail"]
 	if !ok {
 		return err
 	}
-
-	return fmt.Errorf("%w: %s", err, modErr.Detail)
+	return fmt.Errorf("%w: %v", err, detail)
 }