Improve Python setup process

[RunDevelopment]

Fixes #2244
Related to #926.

This improves the Python setup process in a few ways:

1. Fixed a bug where the System Python Path was JSON.parsed. This was a remnant of the old settings system and will always fail now. The system python path will now work correctly.
2. When we fail to find any system python, we fall back to our integrated python. This was bugged because we passed the function of checking python executable the directory the integrated python executable was in and not the path to the python executable.
3. I added a dialog for when installing integrated Python fails. This allows the user to easily retry the installation (e.g. after they ensure their internet connection is stable) or switch to their system python (assuming it is in PATH). This should make troubleshooting a lot more user-friendly.