test: make version based test-extensions failure more explanatory

A buggy __version__ content (usually from setup.py's hg being unable to read the
repo) can make this test fails in an obscure manner. We make the root of the
failure more explicit.

tests/test-extension.t

@@ -946,6 +946,9 b' Older extension is tested with current v'
 Declare the version as supporting this hg version, show regular bts link:
   $ hgver=`$PYTHON -c 'from mercurial import util; print util.version().split("+")[0]'`
   $ echo 'testedwith = """'"$hgver"'"""' >> throw.py
+  $ if [ -z "$hgver" ]; then
+  >   echo "unable to fetch a mercurial version. Make sure __version__ is correct";
+  > fi
   $ rm -f throw.pyc throw.pyo
   $ hg --config extensions.throw=throw.py throw 2>&1 | egrep '^\*\*'
   ** unknown exception encountered, please report by visiting