setup: fail if we cannot determine the version number If running hg fails, exit the setup script unsuccessfully, rather than proceeding to use a bogus version of "+0-". Using an invalid version number causes various tests to fail later. Failing early makes it easier to identify the source of the problem. It is currently easy for setup.py to fail this way since it sets HGRCPTH to the empty string before running "hg", which may often disable extensions necessary to interact with the local repository.