metrics: iperf3: improve error case output

[grahamwhaley]

Currently, if the server container fails we only print out a generic error, as we have already thrown away (not bothered to capture) the output from the 'test connection' command:

12:40:18 Status: Downloaded newer image for gabyct/network:latest
12:40:18 docker pull'd: gabyct/network
12:40:18 Iteration 1
12:40:24 ERROR: iperf server init fails
12:40:24 ERROR: result argument not supplied

See #960

Improve (a little at least), by capturing the output from the last 'test connection' attempt, and echoing it out in case of failure/timeout.