Update windows install prerequisites script

[stefangotz]

The script for installing prerequisites on windows has he following shortcomings:

• it does not work on a fresh msys installation
• it fails to extend PATH so that it can use the python interpreter it installs
• it incorrectly adds the toolchain to PATH
• it does not gracefully handle a failed download of the toolchain zip file
• it references an outdated toolchain zip file

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[projectgus]

Thanks @stefangotz , this looks good and handles most (all?) of the Windows bringup issues mentioned in #200.

Can you please click through and sign the repository's CLA?

[pathob]

@stefangotz ???

[stefangotz]

As I've discussed with @projectgus via e-mail, I'm not comfortable signing the CLA. Sorry about that.