Motivation
Many people do not want to use the integrated python, plus it tends to fail to download.

Description
We should have a message like "Integrated python not found. Would you like to download?" which would allow them to bypass it and turn "use-system-python" on. Then we should also notify after they click that saying "chainner will now install it's required dependencies to your system python installation, would you like to proceed?" With an option to switch back to integrated, just in case.

We also should detect when integrated python fails to download, and show an appropriate error message with a retry option.

Basically we should make it optional but still sorta suggest that the best option is to use integrated, but then also make it so downloading integrated is less likely to cause confusion.

Also, we need to check if the python version is undefined and warn the user that the python instance was corrupt or not found, or something.

Alternatives
At the very least we should warn the user that it failed to download properly rather than silently failing and giving them a cryptic error with no relevant logs.