Add new statistic for exchanges failed but handled in route's MBean

In DEV-3081 the requirement of this new feature was discussed. The ExchangesFailed statistic currently will increment on exchanges that have an exception set. In the case where the failure is handled, the exception will be null but will be stored in an exchange property (IIRC Exchange.EXCEPTION_CAUGHT), so we can check for that to increment a new statistic named like ExchangesFailedAndHandled or ExceptionsThrown