model pose should not be a critical error

[asherikov]

Currently model conversion fails if a pose tag is found in model -> https://github.com/ros/sdformat_urdf/blob/rolling/sdformat_urdf/src/sdformat_urdf.cpp#L99. I believe in this context this is not really an error and conversion should proceed with a warning or completely ignore these tags.

[sloretz]

I could see how a warning would be ok in this case.

I'd recommend against silently ignoring <pose> tags in a <model>. If someone does know that pose is relative to a larger context, they might wonder why it's not included in transforms between frames like map or odom and frames in their model.