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.

discord/errors.py

@@ -38,6 +38,7 @@ __all__ = (
     'ClientException',
     'GatewayNotFound',
     'HTTPException',
+    'RateLimited',
     'Forbidden',
     'NotFound',
     'DiscordServerError',
@@ -137,6 +138,30 @@ class HTTPException(DiscordException):
         super().__init__(fmt.format(self.response, self.code, self.text))
 
 
+class RateLimited(DiscordException):
+    """Exception that's raised for when status code 429 occurs
+    and the timeout is greater than the configured maximum using
+    the ``max_ratelimit_timeout`` parameter in :class:`Client`.
+
+    This is not raised during global ratelimits.
+
+    Since sometimes requests are halted pre-emptively before they're
+    even made, **this does not subclass :exc:`HTTPException`.**
+
+    .. versionadded:: 2.0
+
+    Attributes
+    ------------
+    retry_after: :class:`float`
+        The amount of seconds that the client should wait before retrying
+        the request.
+    """
+
+    def __init__(self, retry_after: float):
+        self.retry_after = retry_after
+        super().__init__(f'Too many requests. Retry in {retry_after:.2f} seconds.')
+
+
 class Forbidden(HTTPException):
     """Exception that's raised for when status code 403 occurs.