Give a better error on Windows if python isn't installed

Before:

```
PS C:\Users\vboxuser\rust> ./x
x.ps1

PS C:\Users\vboxuser\rust>
```

After:
```
PS C:\Users\vboxuser\rust> ./x
x.ps1

C:\Users\vboxuser\rust\x.ps1 : C:\Users\vboxuser\rust\x.ps1: error: did not find python installed
help: consider installing it from https://www.python.org/downloads/windows/
At line:1 char:1
+ ./x
+ ~~~
    + CategoryInfo          : NotInstalled: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,x.ps1
```

The existing message from the shell script is already decent and I decided not to change it:
```
$ ./x
Python was not found but can be installed from the Microsoft Store: ms-windows-store://pdp/?productid=9NJ46SX7X90P
```

0 files changed, 0 insertions, 0 deletions