setup: don't handle missing setuptools_scm gracefully
There's little point in building or installing hg-git, other than to
get the appropriate version markers and dependencies. python ./setup.py build
doesn't do that, and a straight install that way can
lead to a broken setup. Instead, treat it as an hard error.
Fixes #389 (closed)
Edited by Dan Villiom Podlaski Christiansen