Correct documentation regarding 'Check for updates'

When a user starts, lets say… Tails 2.3 (current is 2.11), receives an error message:

file:///usr/share/doc/tails/website/doc/upgrade/error/check.en.html

That says:

This error could also be caused by:
 - an error in Tails Upgrader
 - a failure or an attack on the Tor exit node being used, your 
   Internet service provider, or our website.
Sending us a report is the best way to further analyse the error.

so the user of an old Tails version sends us a report.

We should add to this list something like:

- the version of Tails you are running is too old to do an automatic upgrade, please do a manual upgrade

Related issues

• Related to #12492
• Related to #11131 (closed)
• Related to #12365 (closed)
• Blocks #13423 (closed)

Original created by @emmapeel on 12392 (Redmine)