filesystem: fix error handling

[vtjnash]

walkdir: fix thrown error location and types and performance

• previously the error for the root path would be handled differently than any other error, and errors from isdir would not be passed to the onerror hook
• previously we'd recurse heavily through multiple channels, but there's no reason to do that

readdir: fix thrown error code on some OS

• previously might report the wrong error code (usually just always resulting in saying "unknown error", on e.g. Windows)

seek: fix thrown error code on Windows

• previously would just print a random error reason