`file.read`: unhelpful error when file is missing

[LightAndLight]

When calling file.read on a missing file, I get:

thread 'main' panicked at 'No such file or directory (os error 2)', src/lib.rs:1717:80
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

It's okay for this to crash, but it should print the name of the file, and the ipso source location of the file.read call (https://github.com/LightAndLight/ipso/issues/187).