fix: improve warning message for _error page during static export

[junseokku]

What?

This PR improves the confusing warning message that appears when using a custom _error page during static export (next export).

Changes:

1. packages/next/src/server/render.tsx: Add a specific warning message for _error pages

   • Before: Generic message recommending getStaticProps for all pages
   • After: For _error pages, references the documentation that states getStaticProps is not supported and recommends using 404.js or 500.js instead

2. docs/02-pages/03-building-your-application/01-routing/08-custom-error.mdx: Enhance documentation

   • Clarify that while getStaticProps may technically compile in _error, it has limitations
   • Add guidance on the correct data fetching pattern for static export

Why?

Currently, when running next export with a custom _error page that uses getInitialProps, Next.js displays a warning recommending the use of getStaticProps. However, according to the documentation, _error pages do not support getStaticProps, which causes confusion for developers.

Problem:

• Custom _error page with getInitialProps → warning appears
• Warning message recommends using getStaticProps
• The documentation states that getStaticProps is not supported in _error pages
• Developers experience frustration trying to follow the recommendation that contradicts the documentation

How?

1. Warning message branching:

   • Display a specific warning message when pathname === '/_error'
   • Reference the documentation that states _error pages do not support getStaticProps
   • Recommend using 404.js or 500.js as alternatives

2. Documentation enhancement:

   • Add explanation of technical limitations of getStaticProps in the Caveats section
   • Provide guidance on the correct pattern for static export

[nextjs-bot]

Allow CI Workflow Run

• approve CI run for commit: a7ace7e

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer