nsqd: log errors if no nsqlookupd broadcast address

[ploxiln]

another piece to fix #826

When using nsqlookupd with no explicit broadcast address, and without the fix in #837, it looks like this:

[nsqlookupd] 2016/12/29 13:01:26.395092 CLIENT(127.0.0.1:53473): IDENTIFY Address:plo-pro.local TCP:4150 HTTP:4151 Version:0.3.8
[nsqlookupd] 2016/12/29 13:01:26.395115 DB: client(127.0.0.1:53473) REGISTER category:client key: subkey:
[nsqd] 2016/12/29 13:01:26.395325 LOOKUPD(127.0.0.1:4160): peer info {TCPPort:4160 HTTPPort:4161 Version:0.3.8 BroadcastAddress:}
[nsqd] 2016/12/29 13:01:26.395337 LOOKUPD(127.0.0.1:4160): ERROR - no broadcast address
 ...
[nsqd] 2016/12/29 13:01:27.468701 TOPIC(t1): created
[nsqd] 2016/12/29 13:01:27.468721 ERROR: no available nsqlookupd to query for channels to pre-create for topic t1

I think the ErrList used in GetLookupdTopicChannels() is a bit awkward. It turns into a multi-line error string. And if all lookupd requests experienced an error, it pre-formats it into a plain Error with a multi-line string.

The only other user of GetLookupdTopicChannels(), nsqadmin, ignores the error completely. So maybe the thing to do is have GetLookupdTopicChannels log the errors itself, maybe with a customizable prefix or using a callback ... yeah that sounds pretty messy too, but the current situation could use some kind of refactoring IMHO. Or we could just add these messages and punt :)

[mreiferson]

I think the ErrList used in GetLookupdTopicChannels() is a bit awkward. It turns into a multi-line error string. And if all lookupd requests experienced an error, it pre-formats it into a plain Error with a multi-line string.

I struggled with that, agreed that it's awkward but not sure how important it is to fix.

I think this is a useful addition, thanks.

[mreiferson]

LGTM