[WasmFS][NFC] Assert that error codes are negative

[tlively]

To catch mistakes in backend implementations, assert where possible that error
codes are negative.

[tlively]

Current dependencies on/for this PR:

• main
  • PR [WasmFS] Allow backends to report errors on setSize #17768
    • PR [WasmFS] Catch and report read and write errors in OPFS #17770
      • PR [WasmFS] Make OPFS error handling more robust and regular #17774
        • PR [WasmFS] Allow backends to report errors from flush #17781
          • PR [WasmFS] Report errors from getSize #17783
            • PR [WasmFS][NFC] Small simplification in doMkdir #17784
              • PR [WasmFS] Allow move to return specific error codes #17785
                • PR [WasmFS][NFC] Assert that error codes are negative #17786 👈
                  • PR [WasmFS] Return errors from getEntries and getNumEntries #17807

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