Fix this migration so it won't fail on new installs.

New installs never had an auth_user table, so this will fail in that case, so do it in a try block
instead, so that new installs will be fine.  Also, for new installs, there is no data that needs to be
migrated anyway.
Import databaseerror exception for last commit.
Fix error on new installs in this migration.

Since on new installs auth_user never existed, try to remove them in a try block so that it won't
fail on new installs.