When one or more onion dnode(s) are unavailable when sending privmsg, user gets error msg, but don't have feedback which one it was

[kristapsk]

When there is try to send privmsg to an unavailable message channel, user gets errors like this:

[ERROR]  Tried to communicate on this message channel but failed: onion-network
[ERROR]  You might have to comment out this message channel in joinmarket.cfg and restart.
[ERROR]  No action needed for makers / yield generators!

Problem here is that only feedback user gets is that one of onion network directory nodes has a problem, but it doesn't tell which one.

Source of problem I think is that for old IRC messaging there was MESSAGING:server1, MESSAGING:server2, etc (btw, bad names, why we not switch to MESSAGING:darkscience, MESSAGING:ilita, etc instead?), but for JM native onion messaging it's single MESSAGING:onion messaging channel with multiple directory nodes.

We should output also onion hostname of failing directory node here.