ZkSync: Check balance before operations

[Wiezzel]

Check account balance before making a zkSync account unlock or withdrawal. This allows to display better error messages to the user and avoid panic in the withdraw function (balance and withdraw_fee are of BigUint type so balance - withdraw_fee would cause panic in case the fee is bigger than the account balance).

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Testing intructions:

1. Try init in send mode without funds.

$ cargo run payment init --sender
Error: GenericError { inner: "Not enough balance to unlock account" }  HINT: Did you run `yagna payment fund` and follow the instructions?

2. Try exit without funds.

$ cargo run payment exit
Error: Not enough balance to exit. Minimum required balance to pay withdraw fees is 0.02957 tGLM
$ cargo run payment exit --network=mainnet
Error: Not enough balance to exit. Minimum required balance to pay withdraw fees is 55.36 GLM

3. Regression (init and exit with funds).

[maaktweluit]

LGTM!

One nitty comment, still approving!

[maaktweluit]

Tested this PR with the payment_api examples and yapapi