feat: add status message for 504 caused by optimizer

[sapphi-red]

Description

Failed to load resource: the server responded with a status of 504 (Gateway Timeout)

When Vite responded with 504 that is caused by optimizer, the error above is shown in browser.

This PR changes the status message to make it more understandable.

Additional context

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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 PR Title 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.

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