fix: table.batch should not return errors in the results array (#4507)

Change-Id: I9403a10e16536ac7bea4475e97e2218c48c07866

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Fixes #<issue_number_goes_here> ☕️

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Author: igorbernstein2

bigtable-client-core-parent/bigtable-hbase/src/main/java/com/google/cloud/bigtable/hbase/BatchExecutor.java

@@ -118,7 +118,7 @@ public final void onSuccess(Object message) {
 
     @Override
     public final void onFailure(Throwable throwable) {
-      resultsArray[index] = throwable;
+      resultsArray[index] = null;
       resultFuture.setException(throwable);
     }
   }

bigtable-client-core-parent/bigtable-hbase/src/test/java/com/google/cloud/bigtable/hbase/TestBatchExecutor.java

@@ -257,7 +257,7 @@ public void testPartialResults() throws Exception {
     }
     assertTrue("first result is a result", results[0] instanceof Result);
     assertTrue(matchesRow(expected).matches(results[0]));
-    Assert.assertEquals(exception, results[1]);
+    Assert.assertNull(results[1]); // failed entries null out the result
   }
 
   @Test