Allow configuring the maximum ratelimit timeout before erroring

This is useful for cases where a rate limit is known to be
extraordinarily high, but you still want to handle the error.
This is common with routes such as emoji creation.

docs/api.rst

@@ -4692,6 +4692,9 @@ The following exceptions are thrown by the library.
 .. autoexception:: HTTPException
     :members:
 
+.. autoexception:: RateLimited
+    :members:
+
 .. autoexception:: Forbidden
 
 .. autoexception:: NotFound
@@ -4730,3 +4733,4 @@ Exception Hierarchy
                 - :exc:`Forbidden`
                 - :exc:`NotFound`
                 - :exc:`DiscordServerError`
+            - :exc:`RateLimited`