Make Prepare Special URDF script fail with success exit code

[hello-fazil]

Description

Prepare specialized URDF script fails with error code if unable to find URDF in an export folder which is expected during fresh production installs, stopping the Stretch Install shell scripts. Therefore this PR adds a try exception block to exit the script with success status after notifying the failure.

Testing procedure

• Run stretch_create_ament_workspace.sh with an empty exported_urdf directory.

Before opening a pull request

From the top-level of this repository, run:

• pre-commit run --all-files

To merge

• Squash & Merge

[hello-amal]

LGTM. To follow up on this PR, I'll make the following changes:

1. Run this script at launchtime if the required URDF doesn't exist.
2. Modify this script to use stretch_description/stretch.urdf as opposed to exported_urdf (since the latter may not exist on older robots).