vcpkg manifest mode should check vcpkg.exe existence

vcpkg.exe not exists

```bash
cmake -DCMAKE_TOOLCHAIN_FILE=C:/vcpkg/scripts/builsystems/vcpkg.cmake
```

Actual message:
```
CMake Error at vcpkg/scripts/buildsystems/vcpkg.cmake:288 (message):
   vcpkg install failed
```

I suggest to tell that vcpkg not exist, and tell user to run bootstrap-vcpkg.bat first