cubicweb #17261697 improve FirefoxHelper error message and logging [resolved]

Right now when this command fail to launch firefox it doesn't even display any related error message, for example I have this when I generated the executed command:

$ /home/psychojoker/code/yunohost/ynh-dev/logilab/cubicweb/cubicweb/devtools/data/xvfb-run.sh -a -s -noreset -screen 0 800x600x24 firefox -no-remote -silent --profile /tmp/cwtest-ffxprof-j5w7a6bv -url "http://127.0.0.1:51977/?vid=qunit"

/home/psychojoker/code/yunohost/ynh-dev/logilab/cubicweb/cubicweb/devtools/data/xvfb-run.sh: line 151: tempfile: command not found

Which produce fail tests without any hint of why they have failed.

priorityimportant
typebug
done in3.27.0
load left0.000
closed by<not specified>