[WasmFS] Report errors from getSize

[tlively]

Update the return type of getSize from size_t to off_t, which allows for
negative error codes. off_t is more correct anyway, since in 32-bit mode
it is possible for a file size to be greater than the max value of size_t.
Also update the argument to getSize from size_t to off_t to match.

I manually verified that this error handling works with an injected error, but I
was not able to create a test that could trigger the error naturally.

[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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[tlively]

The browser tests have failed 3 or 4 times in a row on this, but the problem is "too many unresponsive tests," so I think I have to just keep trying until they pass. I'll try running them all locally, too.