Improve error message if not enough memory available for Observability install

[aarthy-dk]

Steps to reproduce

• On Docker Desktop, go to Settings > Resources and set the "Memory limit" to less than 4 GB.
• Install Observability with: python3 dk-installer.py obs install --driver=docker

The first step fails Starting a new minikube profile... FAILED, and the logs have an error like this:

Exiting due to MK_USAGE: Docker Desktop has only 3922MB memory but you specified 4096MB

We should improve the installer to catch this case and display an appropriate message to the user (to either increase memory limit in Docker Desktop or try again with a lower --memory value).