executables' error messages include Rust debug output

[davepacheco]

If you give oxide_controller a bogus config file path, you get a message that includes Rust debug output:

$ ./target/debug/oxide_controller foo
./target/debug/oxide_controller: read "foo": Os { code: 2, kind: NotFound, message: "No such file or directory" }

If you're not a Rust programmer, it's not obvious what this means (and even if you are, the Rust-specific context is more distracting than helpful). It used to look like this:

$ ./target/debug/oxide_controller foo
./target/debug/oxide_controller: read "foo": No such file or directory (os error 2)

which is intended to look like other standard tools. Take GNU grep:

$ grep foo asdf
grep: asdf: No such file or directory

This appears to have been a side effect (that I think was unintended) of 1758338 / #16. I'm going to fix this in #19, as I think it'll be easier to make the tests work if I fix this.

[davepacheco]

It would be even better if we could leave out the "(os error 2)". This would be possible by writing an unsafe wrapper around libc::strerror_r, but I'm not going to do that here.