UserDataPreparer: downgrade an error message from wtf

UserDataPreparerTest#testPrepareUserData_forNewUser_destroysOnFailure
and
UserDataPreparerTest#testPrepareUserData_forExistingUser_doesNotDestroyOnFailure
fail on eng builds because they exercise a code path in UserDataPreparer
that uses Log.wtf, and on eng builds Log.wtf causes the process to
abort.  Fix this by changing the message to ERROR level instead.

Bug: 316035110
Test: atest UserDataPreparerTest
Change-Id: I4dce4db37bf650b7f97e46638c03a429a135a3b6