Slack error page on fluffy image mirroring

[jvperrin]

Sometimes, when slackbridge tries to mirror an image from Slack to fluffy for people on IRC, it'll just mirror a Slack error page (https://failure.ocf.berkeley.edu/administrivia/2019-09-07#23935; and the "image" at https://i.fluffy.cc/nZw3QGxhx9Qf5pWQK5BhgK2HwNPRX81h.html).

For fixing this, not sure if more retries to Slack or something would help, but posting a slack error page to IRC as an "image" certainly isn't helpful.

[cg505]

This seems like a bug on slack's end, but we can try to handle it more gracefully or detect it somehow.