Meetup.com: API errors on RSVPing should return friendlier error messages

[jamietanna]

As per Ryan's comment on chat we should return the human-readable error in the payload to the user

[snarfed]

specifically, here are the example error messages i hit, from 2020-02-17 #indieweb-dev:

• when i try to rsvp to a group's event that i'm not a member of: Error: {"errors":[{"code":"member_error","message":"Invalid member"}]} HTTP Error 400: Bad Request
• when i rsvp to a nonexistent url (that passes the regex): Error: {"errors":[{"code":"group_error","message":"Invalid group urlname xyz"}]} HTTP Error 404: Not Found

the catch is that these are urllib.error.HTTPErrors that propagate all the way up to publish.py, where they're caught and returned as user-visible error messages:

bridgy/publish.py

Line 268 in dea6fac

msg = 'Error: %s %s' % (body or '', e)

the catch is, we want to parse the response body as JSON there and pull out the message field(s) and only show them, but that's specific to the meetup API. so maybe we'd do it here, in granary:

https://github.com/snarfed/granary/blob/d6846f0d1260dd4513464e481c46437cf813c77a/granary/meetup.py#L120-L123