Idempotent Dynamodb2 deletes

Fixes #873

moto/dynamodb2/responses.py

@@ -419,16 +419,12 @@ class DynamoHandler(BaseResponse):
         keys = self.body['Key']
         return_values = self.body.get('ReturnValues', '')
         item = dynamodb_backend2.delete_item(name, keys)
-        if item:
-            if return_values == 'ALL_OLD':
+        if item and return_values == 'ALL_OLD':
             item_dict = item.to_json()
         else:
             item_dict = {'Attributes': {}}
         item_dict['ConsumedCapacityUnits'] = 0.5
         return dynamo_json_dump(item_dict)
-        else:
-            er = 'com.amazonaws.dynamodb.v20120810#ConditionalCheckFailedException'
-            return self.error(er)
 
     def update_item(self):
         name = self.body['TableName']

tests/test_dynamodb2/test_dynamodb_table_with_range_key.py

@@ -314,7 +314,8 @@ def test_delete_item():
     response.should.equal(True)
 
     table.count().should.equal(0)
-    item.delete().should.equal(False)
+    # Deletes are idempotent
+    item.delete().should.equal(True)
 
 
 @requires_boto_gte("2.9")