Follow conversation: Improve error message when request fails due to settings

The API call to follow a conversation in the Reader can return success:false as part of its response. One way to replicate this is by setting WordPress.com's Reader Subscription notification settings to block all emails, and then try to follow a conversation.

When this happen an error message appears that says "Invalid response received". Let's figure out what other cases might produce this error, and then let's see if we can improve this error message and perhaps nudge the user with a path forward. For example, to check their settings to see if they are blocking email.

Start at the Reader's follow-conversation API call and reproduce the failure with Reader Subscription notification settings configured to block all emails. Trace other responses that produce “Invalid response received,” then define an actionable message and verify it covers the identified cases.