Add a check for the specialized URDFs in the launchfile

[hello-amal]

Joint with stretch_install#77 and stretch_production_tools#24.

Although stretch_install#72 changed stretch_create_ament_workspace.sh to run prepare_specialized_urdf.py, that won't actually work since the URDFs don't get generated until after that script is run. As discussed with @hello-fazil and @hello-vinitha , this PR (and its joint PRs, above):

1. Stops running prepare_specialized_urdf.py as part of creating the ROS2 workspace.
2. Instead, adds instructions to run that script as part of the Python SDK QC process.
3. Raises an exception in the web teleop launchfile if the URDFs don't exist, with instructions on how to create them.

Testing

• Delete the URDFs, rebuild your workspace, run ros2 launch stretch_web_teleop web_interface.launch.py and verify it fails and tells you how to generate the URDFs.
• Follow the instructions in the message, re-run ros2 launch stretch_web_teleop web_interface.launch.py, and verify it succeds.