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.
(binary:application/octet-stream)