Added clarification to assert in tester

Author: vbuterin

ethereum/tools/tester.py

@@ -180,7 +180,7 @@ def snapshot(self):
 
     def revert(self, snapshot):
         state_snapshot, txcount, blknum = snapshot
-        assert blknum == self.block.number
+        assert blknum == self.block.number, "Cannot revert beyond block boundaries!"
         self.block.transactions = self.block.transactions[:txcount]
         self.head_state.revert(state_snapshot)