Add an way to check if Tx was successful

[youngjoon-lee]

Requested by Korbit.

We already have a native way:

TxResponse response = client.broadcast(request);
Assert.assertEquals(0, response.getCode());

But, some users don't want to use magic numbers such as 0.

So, let's provide a method to check if tx was successful. CosmJs has a similar way: https://github.com/cosmos/cosmjs/blob/7f7d14630028a35ef213dcffa61c70b35dd5c564/packages/launchpad/src/cosmosclient.ts#L60