Eurotherm: Error when no ramp PID file

As a user setting a ramp on a Eurotherm, I would like to not be alarmed about not having set a ramp PID file if I do not plan on changing PID values.

### Acceptance criteria:

- It is obvious to the user:
- when they have not set a PID file
- that it is not an error if they want to not change PID values with a file

### Notes

- Raised after support call from GEM as they weren't sure if it was a problem