exit bash API-download script immediately if sensor is already running

[joekohlsdorf]

Makes the script do nothing if the sensor is already installed and running, previously the script would fail when run more than once.

This makes the script suitable to be used from AWS SSM State Manager in a simple RunShellScript without using a more complex Distributor Package setup.

Note: This is still not truly idempotent, if the agent is installed but not running the script will still fail because it tries to install a package which is already installed.