TradeInterface exceptions

[Benjamin Cordes]

Functions of tradeinterface have always ApiReponse as a return arg.

public ApiResponse getAvailableBalances(CurrencyPair pair);

getAvailableBalance should have a return argument such as amount, orders, etc. Errors in the request should be handled via exception mechanism, so that errors can be thrown in the hierarchy and handled on various levels.

[Desrever Nu]

Removing milestone: 0.4.2 - 50% code coverage (automated comment)