fix(ssr): improve missing file error

[bluwy]

Description

Fix #10002

When we can't read a file from the fs, transformRequest would return null, hiding the error.

This PR throws the error instead, with an error code to handle an edge case explained below. This would print a descriptive error when a file is missing instead.

Additional context

Partially reverts c30f6d7

The server transform middleware uses transformRequest, but for invalid urls, it needs to skip and let other middlewares handle instead. Skipping is now done by checking for the error code ERR_LOAD_URL.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• 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.

[bluwy]

Putting this as draft as I see the tests being more flaky locally after this change. Gonna make sure it's safe before opening up.

[bluwy]

Did a fresh install and tests are working fine now. Likely some caching issue before