Now support Result responses.

Experimented with the new impl specialization features of Rust. They work! But
they're not quite there yet. Specifically, I was able to specialize on
`Responder`, but when trying to remove the macro in `FromParam`, it didn't work.
See https://github.com/rust-lang/rust/issues/31844.

examples/static_files/src/main.rs

@@ -5,6 +5,7 @@ extern crate rocket;
 
 use rocket::Rocket;
 use std::fs::File;
+use std::io::Error as IOError;
 
 #[route(GET, path = "/")]
 fn index() -> File {
@@ -12,8 +13,8 @@ fn index() -> File {
 }
 
 #[route(GET, path = "/<file>")]
-fn files(file: &str) -> File {
-    File::open(format!("static/{}", file)).unwrap()
+fn files(file: &str) -> Result<File, IOError> {
+    File::open(format!("static/{}", file))
 }
 
 fn main() {

lib/src/response.rs

@@ -5,6 +5,7 @@ use hyper::status::StatusCode;
 use hyper::header;
 use std::io::{Read, Write};
 use std::fs::File;
+use std::fmt;
 
 pub struct Response<'a> {
     pub body: Box<Responder + 'a>
@@ -89,6 +90,30 @@ impl Responder for File {
     }
 }
 
+// Waiting for RFC #1210: impl specialization. It's not quite stable yet.
+// impl<T: Responder, E: fmt::Display + fmt::Debug> Responder for Result<T, E> {
+//     fn respond<'b>(&mut self, res: HypResponse<'b, HypFresh>) {
+//         if self.is_err() {
+//             println!("Response error: {}", self.as_ref().err().unwrap());
+//             return;
+//         }
+
+//         self.as_mut().unwrap().respond(res);
+//     }
+// }
+
+impl<T: Responder, E: fmt::Debug> Responder for Result<T, E> {
+    // prepend with `default` when using impl specialization
+    fn respond<'b>(&mut self, res: HypResponse<'b, HypFresh>) {
+        if self.is_err() {
+            println!("Error: {:?}", self.as_ref().err().unwrap());
+            return;
+        }
+
+        self.as_mut().unwrap().respond(res);
+    }
+}
+
 // TODO: Allow streamed responses.
 // const CHUNK_SIZE: u32 = 4096;
 // pub struct Stream<T: Read>(T);