Give better error message when trying to validate an email for an account that isn't logged in (BB-11012)

steps I took to produce this behaviour:

• in browser BCLOUD-1, create and verify account. remain logged in
• in browser BCLOUD-2, create a second account
• in browser BCLOUD-1, click on the emailed confirmation link for the second account

when I did this (with account BCLOUD-1 being my corporate account, and account BCLOUD-2 being my personal account) I received a message indicating that my confirmation link had expired.

clicking on the confirmation link in browser session BCLOUD-2 was successful.