Remove deprecated .to_err_msg() call

I was getting an error when compiling (against master Rust):
```
src/gen/main.rs:65:42: 65:56 warning: use of deprecated item: use `Show` (`{}` format specifier), #[warn(deprecated)] on by default
src/gen/main.rs:65         Err(x) => fail!("Error: {}\n{}", x.to_err_msg(), usage("glrsgen", opts)),
                                                            ^~~~~~~~~~~~~~
```

Just using `x` fixed the problem.