Fix unit test failure -- file-read wasn't always passing back correct error
codes anymore. If the initial stat() fails (e.g. file doesn't exist or
permission denied), we were double-converting the error code from brackets-shell
format to FS impl API format, causing it to get mangled and treated as
"Unknown."
Merge pull request #9587 from adobe/pflynn/fix-io-error-codes

Fix unit test failure