More informative error for unparsable version
More informative error for unparsable version

When Git is not installed, it is possible to get useless version strings
from versioneer, such as `0+unknown`.  Give a more informative error in
that case.

seberg/numpy,mhvk/numpy,jakirkham/numpy,pdebuyl/numpy,jakirkham/numpy,pdebuyl/numpy,mhvk/numpy,charris/numpy,endolith/numpy,mhvk/numpy,numpy/numpy,seberg/numpy,endolith/numpy,charris/numpy,pdebuyl/numpy,anntzer/numpy,numpy/numpy,anntzer/numpy,anntzer/numpy,mhvk/numpy,mattip/numpy,jakirkham/numpy,pdebuyl/numpy,numpy/numpy,charris/numpy,jakirkham/numpy,seberg/numpy,rgommers/numpy,rgommers/numpy,mattip/numpy,anntzer/numpy,rgommers/numpy,seberg/numpy,numpy/numpy,endolith/numpy,mattip/numpy,mhvk/numpy,charris/numpy,mattip/numpy,rgommers/numpy,endolith/numpy,jakirkham/numpy