Improve error messaging in HollowIncrementalCyclePopulator when Type being removed from IncrementalProducer does not have a primary key.

Previously, a generic NPE would be thrown when trying to construct the HollowPrimaryKeyIndex. Now the offending type will be included in the error.

Author: Dan Smillie

hollow/src/main/java/com/netflix/hollow/api/producer/HollowIncrementalCyclePopulator.java

@@ -105,6 +105,11 @@ private BitSet markTypeRecordsToRemove(HollowReadStateEngine priorStateEngine, f
         HollowSchema schema = priorReadState.getSchema();
         int populatedOrdinals = priorReadState.getPopulatedOrdinals().length();
         if(schema.getSchemaType() == HollowSchema.SchemaType.OBJECT) {
+            HollowObjectSchema objectSchema = (HollowObjectSchema) schema;
+            if (objectSchema.getPrimaryKey() == null) {
+                throw new IllegalArgumentException("Type " + type + " does not have a primary key defined");
+            }
+
             final HollowPrimaryKeyIndex idx = new HollowPrimaryKeyIndex(priorStateEngine, ((HollowObjectSchema) schema).getPrimaryKey()); ///TODO: Should we scan instead?  Can we create this once and do delta updates?
 
             ThreadSafeBitSet typeRecordsToRemove = new ThreadSafeBitSet(ThreadSafeBitSet.DEFAULT_LOG2_SEGMENT_SIZE_IN_BITS, populatedOrdinals);