Activate returns a warning/error code integer

There is no easy way to identify the cause of the error based on the integer. libpcap includes the function pcap_statustostr that accepts an integer and returns the error/warning string associated with it.

It seems like the statustostr function would be a good thing to add.

It it also possible to add an error exception to the activate function, however this may be a breaking change and it would not be faithful to the original libpcap implementation.