[Audio] Friendly handle when Lavalink node is not ready instead of crash

[PredaaA]

Command bugs

Command name

play, and all other Audio commands interacting with the lavalink node.

What cog is this command from?

audio

What were you expecting to happen?

Friendly error like "Can't play tracks at the moment because Lavalink node is not ready." instead of just crashing.

What actually happened?

Traceback (most recent call last):
  File "/home/martinebots/martinebotsvenv/lib/python3.7/site-packages/discord/ext/commands/core.py", line 79, in wrapped
    ret = await coro(*args, **kwargs)
  File "/home/martinebots/martinebotscogs/cogs_modified/audio/audio.py", line 3183, in play
    await self._enqueue_tracks(ctx, query)
  File "/home/martinebots/martinebotscogs/cogs_modified/audio/audio.py", line 3885, in _enqueue_tracks
    result, called_api = await self.music_cache.lavalink_query(ctx, player, query)
  File "/home/martinebots/martinebotscogs/cogs_modified/audio/apis.py", line 911, in lavalink_query
    results = await player.load_tracks(query)
  File "/home/martinebots/martinebotsvenv/lib/python3.7/site-packages/lavalink/rest_api.py", line 300, in load_tracks
    self.__check_node_ready()
  File "/home/martinebots/martinebotsvenv/lib/python3.7/site-packages/lavalink/rest_api.py", line 268, in __check_node_ready
    raise RuntimeError("Cannot execute REST request when node not ready.")
RuntimeError: Cannot execute REST request when node not ready.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/martinebots/martinebotsvenv/lib/python3.7/site-packages/discord/ext/commands/bot.py", line 863, in invoke
    await ctx.command.invoke(ctx)
  File "/home/martinebots/martinebotsvenv/lib/python3.7/site-packages/discord/ext/commands/core.py", line 728, in invoke
    await injected(*ctx.args, **ctx.kwargs)
  File "/home/martinebots/martinebotsvenv/lib/python3.7/site-packages/discord/ext/commands/core.py", line 88, in wrapped
    raise CommandInvokeError(exc) from exc
discord.ext.commands.errors.CommandInvokeError: Command raised an exception: RuntimeError: Cannot execute REST request when node not ready.```

[Drapersniper]

Cheers I'll handle this by raising a user friendly message whenever that RuntimeError is raised by RLL.