Implements std::error::Error for ErrCode.

This change supports "Error interoperation".
See rust-lang/rfcs#201, rust-lang/rust#17747.

Author: gifnksm

src/ffi/err.rs

@@ -1,5 +1,5 @@
 use std::c_str::CString;
-use std::fmt;
+use std::{error, fmt, str};
 use libc::{c_int,c_char};
 
 #[allow(non_camel_case_types)]
@@ -148,3 +148,9 @@ impl fmt::Show for ErrCode {
         }
     }
 }
+
+impl error::Error for ErrCode {
+    fn description(&self) -> &str {
+        unsafe { str::raw::c_str_to_static_slice(curl_easy_strerror(*self)) }
+    }
+}