Use exceptions for addHost()
Gets rid of silly mixed return and only returns the device_id. Throwing an exception if we run into any issues.
Slightly modifies api add host output again to include device_id
Update addHost() to return either device_id or a string
modify print_error(), print_message() to allow for $quiet variable
Only call addHost() once in addhost.php, if the snmp version isn't set, addHost will cope
Fix up snmp-scan.php and discovery-protocols.inc.php to detect when a device already exists
change in ip_exists() to make it more readable
