Incremental restore throws an exception if it is started before full restore

Incremental restore throws NoSuchElementException if it is started before full restore:
```
java.util.NoSuchElementException
        at java.util.TreeMap.key(TreeMap.java:1223)
        at java.util.TreeMap.firstKey(TreeMap.java:284)
        at java.util.TreeSet.first(TreeSet.java:394)
        at com.netflix.priam.backup.IncrementalRestore.execute(IncrementalRestore.java:93)
        at com.netflix.priam.scheduler.Task.execute(Task.java:93)
        at org.quartz.core.JobRunShell.run(JobRunShell.java:199)
        at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:546)
```
Currently there is a AbstractRestore.tracker which aim is to track which files have been already restored and  which are not. But if full restore has not been performed it has empty queue and throws an exception during incremental restoring. As a fix of this issue I decided to add a special check and appropriate error message. Please, review it and let me know if I should change something.