Don't fail when outgoing message amounts sum to more than current balance

[jjcnn]

Scilla uses acceptance semantics, so we should not throw an error just because a contract sends out more money than it has. The error should not occur to until the recipients have accepted more funds than the contract has.

I'm unsure what we should output as the balance in the json output. Several options make sense:

• Output the balance without subtracting anything (this corresponds to the state immediately after the transition finishes)
• Output the balance with the outgoing amounts subtracted, but no less than 0 (this corresponds to the worst-case scenario, because the balance can never go below 0 anyway)
• Output the balance with the outgoing amounts subtracted, even if it is negative (this would make it easier for IDE's to issue the warning that we can't issue from scilla-runner)

[jjcnn]

The tests I've added should be replicated on the C++ side as well, especially for the case where the recipients are user accounts. From Scilla's point of view a user account behaves like a contract that defines the following transition for every possible transition name (as well as the empty string):

transition <name> ()
  accept
end

[jjcnn]

Converting to draft, pending discussions with @AmritKumar and others.

[jjcnn]

Closing, as this PR is superseeded by #1032 .