Wallet: throw exceptions when completing a transaction fails.

If there's insufficient balance (typical error) then InsufficientMoneyException is thrown (checked).
If the SendRequest is bad in some way, like asking to create a spend that would violate the protocol rules, IllegalArgumentException is thrown (unchecked).

Also make VerificationException (and thus ProtocolException+ScriptException) unchecked.

Resolves issue 425.
Increase test coverage.
Add Mike Rosseel to the AUTHORS list