Warn/Error if the decorators aren't properly used

[qianl15]

If you forget to prepend @ to DBOS.step or any DBOS decorator, it will silently not to wrap or register the function.

For example, the following code won't report any error but simply don't checkpoint the step.

DBOS.step()
def step_one():
    DBOS.logger.info("Step one completed!")

@app.get("/workflow")
@DBOS.workflow()
def example_workflow():
    step_one()

Expected behavior: print an error message or warning to the user.