show warning if lockfile doesn't exist on clean

pipenv/core.py

@@ -2302,6 +2302,12 @@ def do_clean(
     # Ensure that virtualenv is available.
     ensure_project(three=three, python=python, validate=False)
 
+    if not project.lockfile_exists:
+        click.echo(
+            '{0}: Could not find Pipfile.lock'.format(crayons.red('Warning'))
+        )
+        sys.exit(1)
+
     installed_packages = delegator.run(
         '{0} freeze'.format(which('pip'))
     ).out.strip().split('\n')
@@ -2344,4 +2350,4 @@ def do_clean(
             # Uninstall the package.
             delegator.run('{0} uninstall {1} -y'.format(which('pip'), apparent_bad_package))
 
-    sys.exit(int(success))
+    sys.exit(int(success))