Standard error codes - QNames or not?

Hello,

I have a question about standard error codes in XProc, such as
'err:XS0001'. Are they meant to be QNames, or just string constants? If
they are QNames, what namespace is the prefix 'err' bound to?

Related to this, can p:error be used for generating standard XProc
errors (such as err:XS0001)?

Regards
Vojtech