return error message from instance to the user

internal/client/client.go

@@ -123,12 +123,33 @@ func (g *Client) sendRequest(method string, url string, requestBody io.Reader, o
 	defer response.Body.Close()
 
 	if response.StatusCode < http.StatusOK || response.StatusCode >= http.StatusBadRequest {
+		message := struct {
+			Error string `json:"error"`
+		}{
+			Error: "",
+		}
+
+		if err := json.NewDecoder(response.Body).Decode(&message); err != nil {
 			return fmt.Errorf(
 				"did not receive an OK response from the GoToSocial server: got %d",
 				response.StatusCode,
 			)
 		}
 
+		if message.Error == "" {
+			return fmt.Errorf(
+				"did not receive an OK response from the GoToSocial server: got %d",
+				response.StatusCode,
+			)
+		}
+
+		return fmt.Errorf(
+			"message received from the instance: (%d) %q",
+			response.StatusCode,
+			message.Error,
+		)
+	}
+
 	if object == nil {
 		return nil
 	}