[SUBS] improvements from demoing the widget

[kasparkallas]

• Instead of showing "Error" when there's a problem with wallet, show "Wallet error" and more clearly instruct the user to try again.
• Listen for contract events for underlying balance and Super Token balance when in wrap & review views. The user might get tokens from another dApp and then that wouldn't reflect in the widget quick enough for the user not to run into a validation error for a low balance.