Improve file not found error message

[sjackman]

When a particular shard file is not found, the error message could be improved:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }', src/libcore/result.rs:1084:5

rust-shardio/src/lib.rs

Line 747 in aa92f23

let mut f = File::open(path).unwrap();