Show a balance error if user doesn't have balance for post-conditions

[jasperjansz]

When signing a transaction in deny mode, we should check whether the user has enough balance to make the transfers subject to post-conditions.

If the post-condition would exclusively cause the transaction to abort, we should show an error. For example, if you have a balance of 10 STX and the post-condition is You'll send exactly 15 STX.

[markmhendrickson]

See Discord for context within MiamiCoin mining case