Improve error message from EB when python is "overshadowed"

[asp8200]

Pardon the poor title for this issue. Feel free to adjust it ;-)

This is a minor thing - we just came across an error message from EB which might be improved.

I was running a pip-installed version of EB, and loaded a GitPython-module (as at one point EB requested support for GitPython), but then I started getting the following error message from any call to EB :

ERROR: No compatible 'python' command found via $PATH (EasyBuild requires Python 2.7+ or 3.5+)

The GitPython-module also loaded a Python-module, which is what caused the problem, but it wasn't clear to me at first since the Python-module was 3.5+.

Link to Slack-thread.