Cosmetic change when printing warnings

Cosmetic change when printing warnings

Warnings can be printed in the following cases:
- When not enough packets are taken into account for measuring packet
latency accuracy.
- When there us a potential network throughput issue, meaning the generator
is generating more packets than the the NIC can handle
- When the generator cannot generate the requested load
If there are no warnings for a certain measurement, nothing gets printed,
suppressing an empty line

Change-Id: Iee07c12142e28dcc0ac406bfed7626731ab08f98
Signed-off-by: Luc Provoost <luc.provoost@intel.com>