Add bucket/cause to errors.MaxConcurrencyReachedError

[owenwastaken]

Summary

Add an attribute to errors.MaxConcurrencyReachedError that stores which bucket (user/channel/guild/global) was reached

Why is this needed?

This attribute would allow people to create custom error messages instead of using a generic "maximum concurrency limit reached" message.

Ideal implementation

To be decided.

Checklist

• I have searched the issue tracker and have made sure it's not a duplicate. If it is a follow up of another issue, I have specified it.