Add !error command

[hexaflexahexagon]

Now that we're actually updating this bot it looks like...

We should add some sort of basic command that displays info relating to the common "help textures are missing" stuff. That is:

mat_error_texture_advanced 2 // this says to replace the pink/black error textures with something else
mat_error_texture_advanced_basetexture dev/reflectivity_60 // this chooses what texture to replace said error materials with. dev/reflectivity_60 is a solid basic choice

We also have this graphic someone made who I can't remember, maybe add that too idk.
https://media.discordapp.net/attachments/235125108656701440/961322110251323482/unknown_1.png

[hexaflexahexagon]

Most likely just needs to be done in #105 whenever that gets done