Revert "Remove the unknown insert validation."

This reverts commit 320669c54e.

app/src/main/java/org/thoughtcrime/securesms/storage/StorageSyncValidations.java

@@ -137,6 +137,10 @@ public final class StorageSyncValidations {
         throw new InsertNotPresentInFullIdSetError();
       }
 
+      if (insert.isUnknown()) {
+        throw new UnknownInsertError();
+      }
+
       if (insert.getContact().isPresent()) {
         SignalContactRecord contact = insert.getContact().get();
 
@@ -169,6 +173,9 @@ public final class StorageSyncValidations {
   private static final class DeletePresentInFullIdSetError extends Error {
   }
 
+  private static final class UnknownInsertError extends Error {
+  }
+
   private static final class MultipleAccountError extends Error {
   }