Error when func.exe is not on PATH.

[t-eckert]

Making a note for myself as I am going to solve this one. On Windows, the user must manually add path/to/func.exe to their PATH environment variable. This is an easy mistake to make. In fact I made it myself and I'm not too dumb.

This fix will check at runtime if the user is on Windows and has path/to/func.exe on PATH. If not, it will give instructions to fix the issue.

I don't think this will take up too much of the compile time.