fix error message in case of missing Python packages

[senko]

This fixes the error message to:

• point to correct location of requirements.txt
• show the name of the package that's missing (in case people need to debug why exactly the import failed)

Demo

Before (notice incorrect location for requirements.txt):

$ python3 main.py
Python environment for Pythagora is not set up.
Please create Python virtual environment: /usr/bin/python3 -m venv /home/senko/Projects/Pythagora/venv
Then install the required dependencies with: /home/senko/Projects/Pythagora/venv/bin/python -m pip install -r /home/senko/Projects/Pythagora/requirements.txt

Now (notice correct location, and reporting of exact package whose import failed):

$ python3 main.py
Python environment for Pythagora is not set up: module `pydantic` is missing.
Please create Python virtual environment: /usr/bin/python3 -m venv /home/senko/Projects/Pythagora/gpt-pilot/venv
Then install the required dependencies with: /home/senko/Projects/Pythagora/gpt-pilot/venv/bin/python -m pip install -r /home/senko/Projects/Pythagora/gpt-pilot/requirements.txt