docs: Improve documentation and error handling for 530 API errors

[uthem150]

Description

This PR improves the documentation and error handling for the HTTP 530 error, making the solution easier for users to understand.

The documentation and error message improvements in this PR are also compatible with the automated fix proposed in #670 and I think it will enhance the user experience regardless of which solution is chosen.

Motivation

The recent 530 errors are a major friction point, and the default error message is cryptic, leaving users confused about how to resolve it.

The goal of this PR is to improve the developer experience by making the error message self-explanatory. With these changes, users can quickly understand the root cause and find the solution in the documentation, regardless of the underlying fix they need to apply.

Changes

1. Documentation Enhancement

   • Added new "Using Custom API Base URL" section to notion-client README
   • Explained how to find the Notion subdomain

2. Error Message Improvement

   • Enhanced error handling for 530 status codes
   • Added actionable error message with solution steps
   • Included link to documentation for reference

3. Test Code Addition

   • Added unit tests to cover the 530 error case.
   • Ensures the new, improved error message is correctly thrown.

Related Issues

• Related to Failed to send https://www.notion.so/api/v3/queryCollection request #669
• Related to Notion Database Gallery cannot be displayed on the browser. transitive-bullshit/nextjs-notion-starter-kit#732

Improved Error Message

[vercel]

@uthem150 is attempting to deploy a commit to the Saasify Team on Vercel.

A member of the Team first needs to authorize it.

[transitive-bullshit]

closed in favor of a simpler solution