Driver, Tracks: Cleanup of leaky types (#20)

Main goal: a lot of nested future/result folding.

This mainly modifies error handling for Tracks and TrackHandles to be
more consistent, and hides the underlying channel result passing in
get_info. Errors returned should be far clearer, and are domain
specific rather than falling back to a very opaque use of the underlying
channel error. It should be clearer to users why their handle commands
failed, or why they can't make a ytdl track loop or similar.

Also fixed/cleaned up Songbird::join(_gateway) to return in a single
await, sparing the user from the underlying channel details and repeated
Errs. I was trying for some time to extend the same graces to `Call`,
but could not figure out a sane way to get a 'static version of the
first future in the chain (i.e., the gateway send) so that the whole
thing could happen after dropping the lock around the Call. I really
wanted to fix this to happen as a single folded await too, but I think
this might need some crazy hack or redesign.

examples/twilight/src/main.rs

@@ -127,10 +127,9 @@ async fn join(msg: Message, state: State) -> Result<(), Box<dyn Error + Send + S
 
     let (_handle, success) = state.songbird.join(guild_id, channel_id).await;
 
-    let content = match success?.recv_async().await {
-        Ok(Ok(())) => format!("Joined <#{}>!", channel_id),
-        Ok(Err(e)) => format!("Failed to join <#{}>! Why: {:?}", channel_id, e),
-        _ => format!("Failed to join <#{}>: Gateway error!", channel_id),
+    let content = match success {
+        Ok(()) => format!("Joined <#{}>!", channel_id),
+        Err(e) => format!("Failed to join <#{}>! Why: {:?}", channel_id, e),
     };
 
     state
@@ -237,7 +236,7 @@ async fn pause(msg: Message, state: State) -> Result<(), Box<dyn Error + Send +
     let store = state.trackdata.read().await;
 
     let content = if let Some(handle) = store.get(&guild_id) {
-        let info = handle.get_info()?.await?;
+        let info = handle.get_info().await?;
 
         let paused = match info.playing {
             PlayMode::Play => {