Added fmt::Display for errors

src/error.rs

@@ -1,4 +1,5 @@
 use std::ffi::NulError;
+use std::fmt;
 
 #[derive(Debug, PartialEq)]
 pub enum Error {
@@ -7,6 +8,16 @@ pub enum Error {
     Unknown
 }
 
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            Error::Io => f.write_str("cannot open file"),
+            Error::Parameter(ref err) => write!(f, "{}", err.as_str()),
+            Error::Unknown => f.write_str("unknown error"),
+        }
+    }
+}
+
 impl From<NulError> for Error {
     fn from(_err: NulError) -> Error {
         Error::Io