Implement Error for SDL Error.

src/sdl2/sdl.rs

@@ -1,3 +1,4 @@
+use std::error;
 use std::ffi::{CStr, CString, NulError};
 use std::fmt;
 use std::rc::Rc;
@@ -28,6 +29,20 @@ impl fmt::Display for Error {
     }
 }
 
+impl error::Error for Error {
+    fn description(&self) -> &str {
+        use self::Error::*;
+
+        match *self {
+            NoMemError => "out of memory",
+            ReadError => "error reading from datastream",
+            WriteError => "error writing to datastream",
+            SeekError => "error seeking in datastream",
+            UnsupportedError => "unknown SDL error"
+        }
+    }
+}
+
 use std::sync::atomic::{AtomicBool, ATOMIC_BOOL_INIT};
 /// Only one Sdl context can be alive at a time.
 /// Set to false by default (not alive).