File Not Found should produce a 404, not a 500

[technosophos]

There is a known limitation in Grain 0.3 (which will be fixed in 0.4) that causes File.pathOpen to cause a WASI error that cannot be trapped by userland code.

When Grain 0.4 is released, File.pathOpen will return a Result, which we can trap and convert to a 404 error.

[technosophos]

As a stopgap, it might be possible to do a directory listing of the parent directory and see if the file is there. If not, return a 404 well before the pathOpen is called.

[technosophos]

Closed by #8