Lease Cmds addLeaseX commands return success when called more than once with the same lease

A user reported issuing the lease4-add command for an existing lease (i.e. more than once in a row) return success and a message stating that the lease was successfully added. In reality, the command handler calls LeaseMgr::addLease4() but ignores the return value, which is false if the lease already exists.

This is bound to lead to confusion for the user. If the subsequent add changed values other than subnet id and address, the user might expect those values to have been updated when in fact they have not. They also could not use the result to know if a lease already exists.

It should return CONTROL_RESULT_ERROR with an explanation that it already exists. This would be consistent with other hook libraries such as subnet cmds and host cmds.