Improve error message on invalid channel config after init timeout

[rbx]

@davidrohr wrote on Nov 16, 2022:

we had runs failing to start with an Error:
fairmq/Device.cxx: LOG(error) << "could not connect all channels after " << fInitializationTimeoutInS << " attempts"; which comes from FairMQ directly.
[...] could you add additional output printing which channel(s) failed to connect?

[rbx]

There is actually already the info as to which channels are still invalid in the debug logs (although it can be quite spammy for many channels and long init times):

[19:51:18][DEBUG] Validating channel 'data[0]'... INVALID
[19:51:18][DEBUG] invalid channel address: 'unspecified'
[19:51:18][DEBUG] Validating channel 'data[0]'... INVALID
[19:51:18][DEBUG] invalid channel address: 'unspecified'
[19:51:18][ERROR] could not connect all channels after 120 attempts

But I agree that it makes sense to list the channels also with the error. I extend the error to:

[19:51:18][ERROR] could not connect all channels after 120 attempts
[19:51:18][ERROR] following channels are still invalid:
[19:51:18][ERROR] channel: name: data[0], type: push, method: connect, address: unspecified

in #455.