fix: print error messages from instance

If an error occurs when sending a request to an instance, try and decode
and print the error message back to the user.

internal/client/client.go

@@ -123,10 +123,25 @@ 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 {
-		return fmt.Errorf(
-			"did not receive an OK response from the GoToSocial server: got %d",
-			response.StatusCode,
-		)
+		message := struct {
+			Error string `json:"error"`
+		}{
+			Error: "",
+		}
+
+		if err := json.NewDecoder(response.Body).Decode(&message); err != nil {
+			return ResponseError{
+				StatusCode:       response.StatusCode,
+				Message:          "",
+				MessageDecodeErr: err,
+			}
+		}
+
+		return ResponseError{
+			StatusCode:       response.StatusCode,
+			Message:          message.Error,
+			MessageDecodeErr: nil,
+		}
 	}
 
 	if object == nil {

internal/client/errors.go

@@ -0,0 +1,36 @@
+// SPDX-FileCopyrightText: 2024 Dan Anglin <d.n.i.anglin@gmail.com>
+//
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package client
+
+import "fmt"
+
+type ResponseError struct {
+	StatusCode       int
+	Message          string
+	MessageDecodeErr error
+}
+
+func (e ResponseError) Error() string {
+	if e.MessageDecodeErr != nil {
+		return fmt.Sprintf(
+			"received HTTP code %d from the instance but was unable to decode the error message: %v",
+			e.StatusCode,
+			e.MessageDecodeErr,
+		)
+	}
+
+	if e.Message == "" {
+		return fmt.Sprintf(
+			"received HTTP code %d from the instance but no error message was provided",
+			e.StatusCode,
+		)
+	}
+
+	return fmt.Sprintf(
+		"message received from the instance: (%d) %q",
+		e.StatusCode,
+		e.Message,
+	)
+}