Fix indexing error that sometimes occurs when deleting users

When a user is deleted around the time that one of its pseuds is being reindexed, the indexing code will sometimes load the pseud, but not be able to find the corresponding user. (They're both deleted in the same transaction, but the pseud is loaded before the user.) This results in a Module::DelegationError when calling user_login to generate the pseud's search document.

Testing Instructions: This is an intermittent bug, so I'm not sure it can be consistently replicated. Probably the best policy with this is to deploy it, and check Resque a few days later to see if the error is still happening.