ci: add error handling for Python venv creation in run.sh

[wangqin0]

I am trying to do a complete ci for my changes following contributing guidelines but it took me a while to discover that my python venv was not setup properly. I added this checking to let it fail faster to save some time for future contributors. I tested this change by remove python3.12-venv package on a Ubuntu-24.04 and here is the output:

┌─[±][no-throw-in-extern-c {1} ✓][root][~/llama.cpp]
└─▪ GG_BUILD_CUDA=1 sudo bash ./ci/run.sh ./tmp/results ./tmp/mnt
The virtual environment was not created successfully because ensurepip is not
available.  On Debian/Ubuntu systems, you need to install the python3-venv
package using the following command.

    apt install python3.12-venv

You may need to use sudo with that command.  After installing the python3-venv
package, recreate your virtual environment.

Failing command: /root/llama.cpp/tmp/mnt/venv/bin/python3

Error: Failed to create Python virtual environment at /root/llama.cpp/tmp/mnt/venv.

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