Question 
Does Israel's failure mean God failed?

Answer 
What if some were unfaithful ? Will their unfaithfulness nullify God's faithfulness? Not at all ! Let God be true, and every human being a liar
Romans 3:3 

It is not as though God's word had failed. For not all who are descended from Israel are Israel. Nor because they are his descendants are they all Abraham's children 
Romans 9:6-7 

Question 
Why are we to blame if God uses our unfaithfulness to accomplish His purposes ?

Answer 
But if our unrighteousness brings out God's righteousness more clearly, what shall we say ? That God is unjust in bringing His wrath on us ? (I am using a human argument.) Certainly not ! 
If that were so, how could God judge the world ? 
Someone might argue, "If my falsehood enhances God's truthfulness and so increases his glory, why am I still condemned as a sinner ?" 
Romans 3:5-7

"One of you will say to me: "Then why does God still blame us ? For who is able to resist his will?' But who are you, a human being, to talk back to God? 
'Shall what is formed say to the one who formed it, Why did you make me like this?
Romans 9:19-20