refactor: Normalize CORS errors

[nzakas]

This pull request introduces a new CorsError class to handle CORS-related errors more effectively and updates various parts of the codebase to use this new error class. The most important changes involve refactoring existing error handling to use CorsError and updating tests accordingly. This ensures that CORS errors all follow the same uniform messaging.

Introduction of CorsError class:

• src/cors.js: Added the CorsError class to represent errors that occur when a CORS request is blocked.

Refactoring error handling to use CorsError:

• src/cors.js: Updated the assertCorsResponse function and methods in the CorsPreflightData class to throw CorsError instead of Error. [1] [2] [3] [4]

Updates to dependent modules:

• src/fetch-mocker.js: Imported CorsError and updated the FetchMocker class to use the new error class in preflight validation and error handling. [1] [2] [3]

Updates to tests:

• tests/fetch-mocker.test.js: Updated tests to expect CorsError instead of Error and adjusted error messages to match the new format. [1] [2] [3] [4] [5]