Better error message when failing to overload metaclass.mro

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-10-15.00:34:04.089>
labels = ['interpreter-core', 'type-feature', '3.7']
title = 'Better error message when failing to overload metaclass.mro'
updated_at = <Date 2017-10-15.00:34:04.089>
user = 'https://github.com/mr-nfamous'

bugs.python.org fields:

activity = <Date 2017-10-15.00:34:04.089>
actor = 'bup'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2017-10-15.00:34:04.089>
creator = 'bup'
dependencies = []
files = []
hgrepos = []
issue_num = 31789
keywords = []
message_count = 1.0
messages = ['304406']
nosy_count = 1.0
nosy_names = ['bup']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue31789'
versions = ['Python 3.7']