Enhancement: UI for failure setting ERC20 balance

[jxom]

Depends on #58

The Request

The ability to see feedback in the UI when a failure occurs while a user is trying to set an ERC20 balance.

Problem

Right now, balanceOf storage slot guessing can fail for some ERC20 tokens that wildly deviate from the ERC20 interface. For these tokens, when a user tries to set a balance for them, no messaging/feedback is provided.

Solution

As a user, when I set a balance for one of my ERC20 tokens on the token list screen, I would expect to see some error messaging (in the form of a toast message) in the UI to tell me that setting a balance failed, and for the input field to become disabled.