Throw a more helpful exception when no nameservers specified/available

My /etc/resolv.conf was wiped by Network Manager (gee, thanks), so
ircd.js was throwing the following unhelpful exceptions when starting
up:
Feb 25 13:47:51 git jsexec[19108]: !JavaScript : uncaught exception: Unable to create any sockets
Feb 25 13:47:51 git jsexec[19108]: !JavaScript : uncaught exception: Unable to create any sockets

First, instead of throwing a string, throw an Error object so we can
actually know where the exception ocurred. Please, everyone, throw Errors
not Strings.

Second, throw a different more helpful exception if there are no nameservers
specified in the DNS constructor or configured on the system.

exec/load/dns.js

@@ -51,6 +51,8 @@ function DNS(synchronous, servers) {
 	if (servers === undefined)
 		servers = system.name_servers;
 
+	if (!servers || !servers.length)
+		throw new Error("No nameservers specified in constructor or configured in system");
 	servers.forEach(function(server) {
 		var sock = new Socket(SOCK_DGRAM, "dns", server.indexOf(':') >= 0);
 		sock.bind();
@@ -63,7 +65,7 @@ function DNS(synchronous, servers) {
 	}, this);
 
 	if (this.sockets.length < 1)
-		throw('Unable to create any sockets');
+		throw new Error('Unable to create any sockets');
 
 	this.increment_id = function() {
 		var ret = nextid;