IOC: Chopper does not display error messages

As an OSIRS instrument scientist I want to see an error if I can not set the frequency of the current chopper. The log shows `Current User Demand Delay Too Large for Angle , Direction & Speed` but this is not reflected in the OPI or IOC. This required debugging by support staff.

### Acceptance Criteria

1. When setting an invalid frequency, or phase the error from the IOC should be shown on the OPI until the next set is issued.

### Notes

The driver can get the error but I am not sure where it see it so this may be hard.

Start by tracing how the chopper driver obtains the error and how the IOC passes it to the OPI; the issue names no files or tests. Reproduce an invalid frequency or phase setting, then verify that the IOC error remains visible on the OPI until another set is issued.

Scritto dal modello di indicizzazione a partire dal testo della issue.