"Nim binary is missing. Build failed." [Win10 x64, nim 0.20.2, old 0.19.4]
## Problem

I unpacked choosenim and tried to run it to install nim 0.20:

```
C:\dnload\nim-etc\choosenim-0.4.0_windows_i386>runme.bat
    Prompt: Can choosenim record and send anonymised telemetry data? [y/n]
        ... Anonymous aggregate user analytics allow us to prioritise
        ... fixes and features based on how, where and when people use Nim.
        ... For more details see: https://goo.gl/NzUEPf.
    Answer: y
Downloading Nim 0.20.2 from nim-lang.org
[#                                                 ] 0mb 0kb/s
      Info: Binary build unavailable, building from source
[##################################################] 100.0% 0kb/s
 Extracting nim-0.20.2.tar.gz
   Building Nim 0.20.2
 Exception: Nim binary is missing. Build failed.
   Cleaning failed build
       Tip: 4 messages have been suppressed, use --verbose to show them.
     Error: Build failed
        1 file(s) copied.
            Work finished.
            Now you must ensure that the Nimble bin dir is in your PATH:
              C:\Users\Mateusz\.nimble\bin
Press any key to continue . . .
```

## Additional info

NOTE: I have a previous installation of Nim 0.19.4 + MinGW active, done without choosenim; it reports as below:

```
C:\dnload\nim-etc\choosenim-0.4.0_windows_i386>nim --version
Nim Compiler Version 0.19.4 [Windows: amd64]
Compiled at 2019-02-01
Copyright (c) 2006-2018 by Andreas Rumpf

git hash: b6d96cafc8bcad1f3d32f2910b25cd11a93f7751
active boot switches: -d:release
```

with `gcc.path = r"C:\dnload\nim-etc\nim\dist\mingw64\bin"` in my `C:\dnload\nim-etc\nim\config\nim.cfg`.

I also have some random ancient gcc in my PATH:

```
C:\dnload\nim-etc\choosenim-0.4.0_windows_i386>gcc --version
2.95
```

It would be super cool if the choosenim tool could report some more details about what's going on under the hood, or maybe log them to some file and print the path to this log file when it starts. As it is now, I find it hard to understand how I could further debug the issue myself :(