Typical behavior of `Channel` is to throw a
`ClosedReceiveChannelException` on invocation of `close` if a `receive`
is suspended. This is unfortunately a bit counter intuitive and can
produce misleading stacktraces.

In order to be more explicit with our exception stacktraces related to
`Channel` cancelation, using `receiveOrNull` and explicitly throwing
`GattClosed` exception when receiving `null`.

May need to revisit this behavior in the future after `receiveOrClosed`
is implemented per Kotlin/kotlinx.coroutines#330. Also noteworthy is
that `receiveOrNull` was undeprecated via Kotlin/kotlinx.coroutines#739.