[commands] Make it so CommandError does not contain @everyone mentions.
[commands] Make it so CommandError does not contain @everyone mentions.

Mitigates some permission elevation issues.

imayhaveborkedit/discord.py,Rapptz/discord.py,Harmon758/discord.py,rapptz/discord.py,mgardne8/discord.py,variablehair/Eggplantato,khazhyk/discord.py,lgkern/PriestPy,Harmon758/discord.py