'ForkedFunc' has confusing failures when attempted under windows

[Winton]

....
INTERNALERROR>   File "c:\python35-x64\lib\site-packages\pytest_forked\__init__.py", line 35, in pytest_runtest_protocol
INTERNALERROR>     reports = forked_run_report(item)
INTERNALERROR>   File "c:\python35-x64\lib\site-packages\pytest_forked\__init__.py", line 55, in forked_run_report
INTERNALERROR>     ff = py.process.ForkedFunc(runforked)
INTERNALERROR>   File "c:\python35-x64\lib\site-packages\py\_process\forkedfunc.py", line 45, in __init__
INTERNALERROR>     pid = os.fork()
INTERNALERROR> AttributeError: module 'os' has no attribute 'fork'

https://ci.appveyor.com/project/nooperpudd/quantbube/build/1.0.38/job/7pdjtq6p78hu2127

https://github.com/pytest-dev/pytest-forked/issues/6

[Ronny Pfannschmidt]

windows has no fork support

[Ronny Pfannschmidt]

actually, we should error better there