Add LoginFailure exception for a clearer failure in Client.login

The older HTTPException is not exactly the clearest thing for people
who are new to programming or HTTP exceptions in general.

discord/errors.py

@@ -88,3 +88,10 @@ class InvalidArgument(ClientException):
     :exc:`DiscordException`.
     """
     pass
+
+class LoginFailure(ClientException):
+    """Exception that's thrown when the :meth:`Client.login` function
+    fails to log you in from improper credentials or some other misc.
+    failure.
+    """
+    pass