Give a proper error message when the hg repo is not known

hg-fast-export.sh always passes the --repo flag to hg-fast-export.py.
If, for some reason, we have a state file where the repo-url is an
empty string the checks in hg-fast-export.py will not work and the
user will be confused. Therefore we check that the url is specified
before calling hg-fast-export.py.