Make nox -s run emit a helpful error if no command invocation is passed

noxfile.py

@@ -201,8 +201,11 @@ def build_help(session):
 
 
 def _run(session):
- """Run an arbitrary command in the project's venv."""
- session.run(*session.posargs[1:])
+ """Run an arbitrary command invocation in the project's venv."""
+ posargs = session.posargs[1:]
+ if not posargs:
+ session.error("Must pass a command invocation to run")
+ session.run(*posargs)
 
 
 @nox.session()