Implement std::fmt::Display for the error types

examples/show.rs

@@ -1,9 +1,12 @@
 extern crate libnotify;
 
 fn main() {
-    let notify = libnotify::Context::new("hello").unwrap();
+    let notify = libnotify::Context::new("hello").unwrap_or_else(|e| {
+        panic!("{}", e);
+    });
+    let body_text = Some("This is the optional body text.");
     let n = notify.new_notification("This is the summary.",
-                                    Some("This is the optional body text."),
-                                    None).unwrap();
-    n.show().unwrap();
+                                    body_text,
+                                    None).unwrap_or_else(|e| panic!("{}", e));
+    n.show().ok().expect("Failed to show notification");
 }