Prevent script failure in getLifetimeCredits()

Sometimes, BingRewards fails with the message:
   s = page.index('<div class="data-lifetime')
   ValueError: substring not found

This change allows the script to continue, despite not being able to find the lifetime credits count for a given account.  I'm not sure why the failure occurs, but this way it can at least keep running (since being unable to find lifetime credits isn't something that should end the script).