Make error messages more specific when users fail to post to an unscoped Publisher group

**Describe the bug**
Right now if you attempt to save an annotation in a unscoped Open publisher group of which you're not a member, you get a generic "Error: Saving annotation failed".

This message doesn't explain why you can't save the message (i.e., you're not a group member and this is an Open publisher group), and for a user who hasn't seen other error messages isn't distinguishable from an expected error message for being offline, or lacking some other permission.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to https://www.theguardian.com/environment/2020/jun/13/climate-worst-case-scenarios-clouds-scientists-global-heating#annotations:group:ZLx677oM
2. Select text, choose "Annotate, and attempt to Post an annotation
3. See error

**Expected behavior**
The error message should contain text like "You do not have permission to post in this group", perhaps with a link or tool tip explaining Open groups.

**Additional context**
https://hypothesis.zendesk.com/agent/tickets/10803

> If I can pass along a suggestion related to the other problem we ran into, maybe there could be a more specific error message for users who attempt to post to a group they aren't part of? And perhaps it could even redirect them to posting in public? Otherwise we're feeling a little hesitant to use the group link right now as it might dead-end a reviewer who decided to annotate but hadn't cleared their login with us, or that we'd forgotten to request to be added to the group. As always, relying on volunteered efforts makes us extremely user-friction-averse.

Start by reproducing the annotation-posting failure using the linked Guardian article and determine where the generic saving error is produced. The work is done when a non-member posting to an unscoped Open publisher group sees a specific permission message, optionally with the proposed group guidance or link.

Written by the indexing model from the issue text.