Improve disk full error message

[sjackman]

When the disk is full, the error message could be improved.

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "SendError(..)"', src/libcore/result.rs:1084:5

rust-shardio/src/lib.rs

Line 705 in aa92f23

self.tx.send(Some(send_buf)).unwrap();