Extract API error logic to helper

The submit command handles API errors correctly. We need to be
able to share this logic so we can stop swallowing errors in
other commands.

cmd/cmd.go

@@ -1,7 +1,9 @@
 package cmd
 
 import (
+	"encoding/json"
 	"fmt"
+	"net/http"
 
 	"io"
 
@@ -70,3 +72,21 @@ func validateUserConfig(cfg *viper.Viper) error {
 	}
 	return nil
 }
+
+// decodedAPIError decodes and returns the error message from the API response.
+// If the message is blank, it returns a fallback message with the status code.
+func decodedAPIError(resp *http.Response) error {
+	var apiError struct {
+		Error struct {
+			Type    string `json:"type"`
+			Message string `json:"message"`
+		} `json:"error,omitempty"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&apiError); err != nil {
+		return fmt.Errorf("failed to parse API error response: %s", err)
+	}
+	if apiError.Error.Message != "" {
+		return fmt.Errorf(apiError.Error.Message)
+	}
+	return fmt.Errorf("unexpected API response: %d", resp.StatusCode)
+}

cmd/submit.go

@@ -2,7 +2,6 @@ package cmd
 
 import (
 	"bytes"
-	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -254,12 +253,7 @@ func (s *submitCmdContext) submit(metadata *workspace.ExerciseMetadata, docs []w
 	defer resp.Body.Close()
 
 	if resp.StatusCode == http.StatusBadRequest {
-		var jsonErrBody apiErrorMessage
-		if err := json.NewDecoder(resp.Body).Decode(&jsonErrBody); err != nil {
-			return fmt.Errorf("failed to parse error response - %s", err)
-		}
-
-		return fmt.Errorf(jsonErrBody.Error.Message)
+		return decodedAPIError(resp)
 	}
 
 	bb := &bytes.Buffer{}
@@ -430,10 +424,3 @@ func (s submitValidator) isRequestor(metadata *workspace.ExerciseMetadata) error
 func init() {
 	RootCmd.AddCommand(submitCmd)
 }
-
-type apiErrorMessage struct {
-	Error struct {
-		Type    string `json:"type"`
-		Message string `json:"message"`
-	} `json:"error,omitempty"`
-}