Miniconda Installer (Windows) - handle missing/corrupted Anaconda installation

[obarak]

Orange version

3.8-Win64 (Installer file name is Orange3-3.8.0-Miniconda-x86_64.exe)

Actual behavior:

1. I've downloaded and ran the Miniconda installer.
2. Installer detected that I already have Anaconda installed. Installation continued and completed with no error messages. Only that...
3. ...Orange wouldn't start (nothing happens when clicking on the shortcut).
4. Making a long story short - turns out I had two Anacondas installed on my machine:
   • An old one that was completely removed from everywhere except for the Windows Registry (there were no physical files on disk). That was the one Orange installer tried to use.
   • A new one, working perfectly, but undetected by the installer.
5. Turns out that corrupted Anaconda installations on Windows are actually quite common, due to Anaconda's broken uninstaller and backward incompatibility issues.
6. Examining the Orange installer log revealed the following lines:

Creating a conda env in "C:\Users\obarak\AppData\Local\Orange"
The system cannot find the path specified.

Last line is an error but, again, it didn't cause any error message to appear.
7. Removing the orphaned Anaconda registry key and then re-installing solved the issue.

Expected behavior:

Ideally, if the installer can't find the conda.exe file it needs for installation, it will try other Anacondas available on the machine (if exist), and finally revert to installing Miniconda from scratch.
Less ideal, but still good - show an error to the user and cancel installation.

[astaric]

The installers were modified in #2827 to do a basic validation of the conda environment (they check for presence of python and conda executable). If they are not found, a new miniconda installation is forced (not ideal, but should avoid the error)

The new installers will be built when 3.9 is released (probably later this month).