SET ROLE and reserved roles

Hi,

I observe this:

postgres=# SET ROLE TO NONE;
SET

postgres=# SET ROLE TO nonexistent;
ERROR:  role "nonexistent" does not exist

postgres=# SET ROLE TO pg_signal_backend;
ERROR:  invalid value for parameter "role": "pg_signal_backend"

Is that behavior deliberate? Might it be better to handle the case
specially much as setting to "none" works?  Such as:

ERROR: cannot set to reserved role "pg_signal_backend"

Sorry if I have missed any discussion where such a choice was deliberately
made.

Thanks,
Amit