Handle exceptions from cursor.next in ReplSetTest

The `checkOplogs` function in ReplSetTest assumes that if a cursors `hasNext()` function does not throw an error that that cursors `next()` function will also not throw an error. This is not true, however. There are some cases where `hasNext()` could return true and `next()` throws an error. The `checkOplogs` function should handle those cases.