Change ERROR GetModelProperties to a Warn?

[chcorbato]

I leave this open because maybe you want to change
[ERROR] [1589867337.469781183, 6.110000000]: GetModelProperties: model [unknown_obstacle1] does not exist
into a WARN, but feel free to close it or triage it as you see it fit.

I see this is not an issue of the script for the experiments but an error from gazebo probably raised here (to be confirmed)

metacontrol_experiments/scripts/create_unknown_obstacles.py

Line 34 in 2e0f679

check_obstacle_response = check_obstacle_service(obstacle_name)

Originally posted by @chcorbato in #4 (comment)