Log channel exceptions instead of just swallowing them
Don't encode anything if outgoing message is too long

This avoids an extra exception which happened because the too-long-frame exception already triggered closing the channel, but then we try to send an empty frame in it's place, and get an exception because the channel is already closed so we can't send anything.
Configure load tester netty logging to go to SLF4J

Netty is configured by default to send its logging messages through JDK logging, but nifty-load-tester uses SLF4J