Tutorial Extension: On-chain Balance check

[ggwpez]

We could add an extension for on-chain balance checks.
This would also prevent a user from using a wrong mnemonic in ganache, since we could check the balance before sending the first TX.
Otherwise the ganache chain just prints eth_getTransactionReceipt forever until it times out.

This came up as an idea after getting feedback for our tutorial code from a user who had this problem.