Add a meaningful message after withdraw

[andreipope]

Below message is displayed after withdraw command is executed:

Withdrawing db0d1408e211fc91
Withdrew [object Object]

There is no easy way to find out is the operation was successful or not. Can we have a meaningful message instead?

[gakonst]

Should make https://github.com/loomnetwork/loom-js/blob/master/src/plasma-cash/entity.ts#L491 to also setup a Withdrew event listener so that it logs success.