propagate errors in ClientManager.destroy (#844)
propagate errors in ClientManager.destroy (#844)

If the promise returned by logout() rejects, previously
it would be completely uncaught, and just return an
eternally pending promise that never resolved.

Related to pull requests #828 and #839.
zajrik/discord.js,SpaceEEC/discord.js,Lewdcario/discord.js,hydrabolt/discord.js,CodeMan99/discord.js,robflop/discord.js,Drahcirius/discord.js,aemino/discord.js,SpaceEEC/discord.js,SpaceEEC/discord.js,zajrik/discord.js,Drahcirius/discord.js,aemino/discord.js,appellation/discord.js,robflop/discord.js,CodeMan99/discord.js,Lewdcario/discord.js,devsnek/discord.js,hydrabolt/discord.js,aemino/discord.js,Lewdcario/discord.js,Drahcirius/discord.js,hydrabolt/discord.js,zajrik/discord.js,appellation/discord.js,robflop/discord.js,appellation/discord.js,devsnek/discord.js,CodeMan99/discord.js,devsnek/discord.js