Add better user feedback for expired install token

[postmastersteve]

When users have a ProPhoto Installer plugin which was downloaded too long ago, the installer will fail to load ProPhoto onto the site. When it fails for this reason, the user gets a very generic message:

Sorry, something went wrong downloading the latest version of P7. Please refresh this page and try again. If the problem continues, please contact us for more help.

When the install fails for the reason of an outdated token, we should provide instructions for the user to remove the plugin and load a fresh copy from https://pro.photo/me

[csauve]

+1 to this. I couldn't figure out the cause of the error until I found this issue.