mac: Add ParseError enum

The `mac::ParseError` enumeration is intended to represent all of the
possible ways `MacAddress::from_string` could fail. For now, the only
known problem is an invalid hexadecimal integer found in one of the
octets, which causes a `ParseIntError`. The `ParseError` enum implements
the `Debug`, `Display`, and `Error` traits.

src/mac.rs

@@ -1,13 +1,46 @@
+use std::error;
+use std::fmt;
 use std::num;
 
 
+#[derive(Debug)]
+pub enum ParseError {
+    Value(num::ParseIntError),
+}
+
+
+impl From<num::ParseIntError> for ParseError {
+    fn from(e: num::ParseIntError) -> Self {
+        ParseError::Value(e)
+    }
+}
+
+
+impl fmt::Display for ParseError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            ParseError::Value(ref e) => e.fmt(f),
+        }
+    }
+}
+
+
+impl error::Error for ParseError {
+    fn cause(&self) -> Option<&error::Error> {
+        match *self {
+            ParseError::Value(ref e) => Some(e),
+        }
+    }
+}
+
+
 pub struct MacAddress {
     addr: [u8; 6],
 }
 
 
 impl MacAddress {
-    pub fn from_string(s: &str) -> Result<Self, num::ParseIntError> {
+    pub fn from_string(s: &str) -> Result<Self, ParseError> {
         s.split(":")
             .map(|b| u8::from_str_radix(b, 16))
             .collect::<Result<Vec<u8>, _>>()
@@ -15,7 +48,8 @@ impl MacAddress {
                 let mut addr = [0u8; 6];
                 addr.copy_from_slice(&parts[..6]);
                 Ok(MacAddress { addr })
-        })
+            })
+            .map_err(ParseError::from)
     }
 
     pub fn as_bytes(&self) -> &[u8; 6] {