Since 3.6.0 sipp is throwing a warning that doesn't
change behavior but we detect an error file as failed
sipp execution for responders
So let's clean the known error from error files and
remove the empty ones after
See details at https://github.com/SIPp/sipp/issues/497
* bin/sipp.sh: add -e option to define error file
* bin/check.sh: use -e option with the proper filename
so there's no need to move the files later
Change-Id: I5ce4b73d858d928f4bc9ec89bfe8658fad48b6e6