Reader: display follow errors in the appropriate position

[designsimply]

Raised by @bluefuton

When a follow or unfollow error is encountered, we always show the error on the post card:

...and it persists in the full post view if the user doesn't dismiss it.

This sometimes feels a bit odd. We should perhaps store the origin of the follow, so we can display the error in the appropriate places.

Note from @designsimply

Error for reference:

Sorry, we couldn't follow this site. Please try again in a while. 

[blowery]

Need to determine if this is still a problem

[blowery]

Yup, still a problem. Moving to feature backlog.

[alisterscott]

The error notice seems to appear at the very top now regardless:

And it doesn't seem to follow to full post view.

Closing this and we can raise a separate issue if the current display is not correct.