Investigate and fix random test failure

[lovasoa]

Handle database "table doesn't exist" errors in FileSystem to prevent test failures due to race conditions and ensure fallback to embedded files.

The basic::test_index_ok test sometimes fails because it attempts to read index.sql from the database when the sqlpage_files table has been concurrently dropped by another test. Previously, the FileCache would receive a database error (e.g., "Table 'sqlpage.sqlpage_files' doesn't exist") and fail, instead of interpreting it as "file not found in database" and falling back to serving the embedded index.sql from static_files. This change explicitly catches these database errors and converts them into a NotFound error, allowing the correct fallback mechanism to engage.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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