Fixed issue where child tx failure wouldn't result in vm throwing TransactionFailed

If you had something like this 

```def foo():
   x = self.bar()
   return(x)

def bar():
   ~invalid()```

Calling self.foo() in pyethereum would *not* result in a transaction failure but instead just return 0

2f8efd7 it last worked in that commit

I think it was broken sometime before this commit : 0a7417f

This commit I'm making here fixes it (there may be a better more elegant way of fixing it, but at least this fix makes it so all the contracts that use this pattern are no longer broken).

Author: joeykrug

ethereum/vm.py

@@ -637,7 +637,7 @@ def vm_execute(ext, msg, code):
                 # Get result
                 result, gas, data = ext.msg(call_msg)
                 if result == 0:
-                    stk.append(0)
+                    return vm_exception('Child Call Failed')
                 else:
                     stk.append(1)
                 # Set output memory