Properly handle python.exe xor py being present on windows

[konstin]

We currently assume py, the python launcher on windows, is always present. We should test without it and allow only python.exe and show good error messages when both are missing.

[mjclarke94]

There's also python-launcher for unix which also provides a py command (and is waaaay faster than pyenv et al). Might be worth tying it to the presence of a py command in the Path rather than just Windows.

[T-256]

Related #1521