Change Error from a struct to an enum (BREAKING CHANGE).

This allows us to separate out the underlying SQLite error codes from
errors that occur on the Rust side.

libsqlite3-sys/src/error.rs

@@ -1,4 +1,5 @@
 use libc::c_int;
+use std::error;
 use std::fmt;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -77,6 +78,12 @@ impl fmt::Display for Error {
     }
 }
 
+impl error::Error for Error {
+    fn description(&self) -> &str {
+        code_to_str(self.extended_code)
+    }
+}
+
 // Result codes.
 
 pub const SQLITE_OK        : c_int =   0;