docs: delegate error handling to express’s error handler

[dtinth]

Description

This PR updates the SSR example code to pass errors to Express.js’s error handler through the next argument, instead of handling it directly from the endpoint, for more idiomatic Express code.

Additional context

N/A

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[Shinigami92]

@import-brain you said you have express knowledge, could you review that?

[import-brain]

@Shinigami92 lgtm