Improve when and how Makefile suggests virtual env

The avoids showing the message when the build command was already
run in a virtual environment.

It also keeps the command failing, so the subsequent twine command
is not attempted. (Just adding "|| echo ..." caused the command to
succeed, because "echo ..." itself succeeds except in the rare case
it cannot write to standard output.)

Makefile

@@ -21,6 +21,13 @@ release: clean
 force_release: clean
 	# IF we're in a virtual environment, add build tools
 	test -z "$$VIRTUAL_ENV" || pip install -U build twine
-	python3 -m build --sdist --wheel || echo "Use a virtual-env with 'python -m venv env && source env/bin/activate' instead"
+
+	# Build the sdist and wheel that will be uploaded to PyPI.
+	python3 -m build --sdist --wheel || \
+		test -z "$$VIRTUAL_ENV" && \
+		echo "Use a virtual-env with 'python -m venv env && source env/bin/activate' instead" && \
+		false
+
+	# Upload to PyPI and push the tag.
 	twine upload dist/*
 	git push --tags origin main