fix(handlers): Fix chained error handling in actors

This took me some time to figure out so it's useful to document in the
commit message.

When chaining messages from actors, the result type of a message (i.e.
the actual `<M as Message>::Result`) is sometimes itself a
`Result<T, E>`.

In many cases this leads to a situation where the return type of a
message sending process is something like (simplified):

    Future<Item=Result<Foo, ConverseError>, Error=actix::MailboxError>

Due to the implementation of
`From<actix::MailboxError> for ConverseError` it is possible to use
`.from_err()` on these futures to convert the future's `Error` to
`ConverseError`.

The type `Result` apparently implements `IntoFuture`, which means that
due to some trait magic that's been applied somewhere in the futures
API a call to `flatten()` can "lift" the contained error if the error
types match and give us a "simple"

    Future<Item=Foo, Error=ConverseError>

From that point on chaining becomes easy.

src/handlers.rs

@@ -34,9 +34,12 @@ pub struct AppState {
 
 pub fn forum_index(state: State<AppState>) -> ConverseResponse {
     state.db.send(ListThreads)
-        .and_then(move |res| state.renderer.send(IndexPage { threads: res.unwrap() }))
-        .from_err()
-        .map(|res| HttpResponse::Ok().content_type(HTML).body(res.unwrap()))
+        .flatten()
+        .and_then(move |res| state.renderer.send(IndexPage {
+            threads: res
+        }).from_err())
+        .flatten()
+        .map(|res| HttpResponse::Ok().content_type(HTML).body(res))
         .responder()
 }
 
@@ -44,22 +47,20 @@ pub fn forum_index(state: State<AppState>) -> ConverseResponse {
 pub fn forum_thread(state: State<AppState>, thread_id: Path<i32>) -> ConverseResponse {
     let id = thread_id.into_inner();
     state.db.send(GetThread(id))
-        .and_then(move |res| {
-            let u = res.unwrap();
-            state.renderer.send(ThreadPage {
-                thread: u.0,
-                posts: u.1,
-            })
-        })
-        .from_err()
-        .map(|res| HttpResponse::Ok().content_type(HTML).body(res.unwrap()))
+        .flatten()
+        .and_then(move |res| state.renderer.send(ThreadPage {
+            thread: res.0,
+            posts: res.1,
+        }).from_err())
+        .flatten()
+        .map(|res| HttpResponse::Ok().content_type(HTML).body(res))
         .responder()
 }
 
 /// This handler presents the user with the "New Thread" form.
 pub fn new_thread(state: State<AppState>) -> ConverseResponse {
-    state.renderer.send(NewThreadPage).from_err()
-        .map(|res| HttpResponse::Ok().content_type(HTML).body(res.unwrap()))
+    state.renderer.send(NewThreadPage).flatten()
+        .map(|res| HttpResponse::Ok().content_type(HTML).body(res))
         .responder()
 }