Add ApiException class for proper Error inheritance

[TaprootFreak]

Summary

Replace plain ApiError objects with ApiException class that extends Error, so error handlers can properly display the message.

Problem

The previous implementation threw plain objects {statusCode, message}. When these objects were caught by error handlers (webpack-dev-server overlay, browser console), they were displayed as [object Object] instead of the actual error message.

Solution

1. ApiException class

export class ApiException extends Error implements ApiError {
  constructor(
    public readonly statusCode: number,
    message: string,
  ) {
    super(message);
    this.name = 'ApiException';
    Object.setPrototypeOf(this, ApiException.prototype);
  }
}

Features:

• Extends Error → proper .message, .stack, toString()
• Implements ApiError → backward compatible
• Object.setPrototypeOf → ES5 prototype chain fix

2. Network error handling

.catch((error: unknown) => {
  const message = error instanceof Error ? error.message : String(error);
  throw new ApiException(0, `Network error: ${message}`);
})

3. API error handling with fallbacks

return response.json()
  .catch(() => null)
  .then((body: Partial<ApiError> | null) => {
    throw new ApiException(
      body?.statusCode ?? response.status,
      body?.message ?? response.statusText ?? 'Unknown error',
    );
  });

Edge cases handled:

Server Response	statusCode	message
{"statusCode": 400, "message": "Error"}	400	"Error"
{"error": "Bad"}	response.status	response.statusText
["error"]	response.status	response.statusText
"string"	response.status	response.statusText
null	response.status	response.statusText
HTML/invalid JSON	response.status	response.statusText

Test Plan

• Network error → Network error: Failed to fetch
• API error → actual error message
• Non-JSON response → HTTP status text
• error.statusCode works
• error.message works
• error instanceof Error → true
• error instanceof ApiException → true