Missing "rolls" in loot tables causes NullPointerException

How to reproduce

1. Try to load the following loot table:

   {
     "pools": [
       {
         "entries": [
           {
             "type": "item",
             "name": "stone"
           }
         ]
       }
     ]
   }

1. The loot table doesn't load and a NullPointerException is thrown, but luckily caught due to the fix for MC-91061.

This not a friendly message. Either it should explain the problem, or it should treat a missing rolls parameter as "rolls": 1.