Subscriptions: Confirmation link in email gives error if logged into WP.com with different account

Steps to reproduce the issue:
1. Log in to WordPress.com.
2. Find a JP connected blog with subscriptions active. Subscribe using a different email address than what you're logged into wp.com with.
3. Check that email address and click the confirmation link.
You will likely see something like this:

Error message (Updated by @htdat so it can be indexed and searched):

> Your subscription could not be activated. It may have expired, or the email address you subscribed with is not attached to your WordPress.com account.


The Url of the page I'm visiting still contains the correct email address I used to subscribe:
(jetpackjacobwp@gmail.com)

But due to wp.com seeing me logged in with the primary account, I'm unable to manage the subscriptions for that other address.
#### What I expected

To be taken to the subscription management for the email address where I received the confirmation email.
#### What happened instead

I was taken to the subscription management for the WP.com account I was logged into instead.

Other notes:
Not sure there's much to be done about this since it's opening in wp.com/calypso, and you wouldn't want one account to have the ability to manage subscription permissions over a separate account, but we may want to expand the error messaging with suggestions like "Log out of WordPress.com and click the confirmation link again" or "Try in an incognito window/separate browser"

Start by reproducing the confirmation flow with a WordPress.com account different from the subscription email, then inspect the WordPress.com/Calypso handling of the confirmation URL and the displayed error. Done means the user can reach the correct subscription management view, or receives clear guidance to log out or retry in a separate browser when account access prevents that.

Written by the indexing model from the issue text.