network: move logging from tc .fill_message to the callers

Structured initialization is used a bit more.

There were two kinds of log messages: about failed size calculations and
about failed appends to the message. I "downgraded" the first type to log_debug,
and moved the latter to the caller. This way there should be at most one high-priority
message.

I also changed sizeof(<type>) to sizeof(var) — there is less chance of select-and-paste
error in the second form.