Log errors if/when comWriteBuf() fails.

This is to hopefully help to root-cause or even possibly resolve the issue
reported in #synchronet by PyRoDrAkE:
"It all works well until the Dialup connection runs slower than 4800 baud
using SexPots. I connect from my XT at either 1200 baud or 2400 baud and it
would just stop drawing midway."

There shouldn't be any write timeout in comWriteBuf(), so I don't know why
this would help, but it's worth a try. Logging any comWriteBuf() errors is
also a good idea in general.