trex: Improve error handling

trex: Improve error handling

Print an error message in case, that vsperf is not able
to connect to the T-Rex server. User is instructed
to check if T-Rex server is up and running.

Change-Id: I70c6d27fa8331921d43e25c5eaa3638ded6c015b
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Jose Lausuch <jalausuch@suse.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>
Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>