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.

src/driver/connection/error.rs

@@ -11,7 +11,11 @@ use xsalsa20poly1305::aead::Error as CryptoError;
 
 /// Errors encountered while connecting to a Discord voice server over the driver.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum Error {
+    /// The driver hung up an internal signaller, either due to another connection attempt
+    /// or a crash.
+    AttemptDiscarded,
     /// An error occurred during [en/de]cryption of voice packets or key generation.
     Crypto(CryptoError),
     /// Server did not return the expected crypto mode during negotiation.
@@ -83,6 +87,7 @@ impl fmt::Display for Error {
         write!(f, "Failed to connect to Discord RTP server: ")?;
         use Error::*;
         match self {
+            AttemptDiscarded => write!(f, "connection attempt was aborted/discarded."),
             Crypto(c) => write!(f, "cryptography error {}.", c),
             CryptoModeInvalid => write!(f, "server changed negotiated encryption mode."),
             CryptoModeUnavailable => write!(f, "server did not offer chosen encryption mode."),