The back ends should
use the function nnheader-report to report error
conditions—they should not raise errors when they aren't
able to perform a request. The first argument to this function is
the back end symbol, and the rest are interpreted as arguments to
format if there are multiple of them, or just a
string if there is one of them. This function must always returns
nil.
(nnheader-report 'nnchoke "You did something totally bogus")
(nnheader-report 'nnchoke "Could not request group %s" group)
Gnus, in turn, will call nnheader-get-report when
it gets a nil back from a server, and this function
returns the most recently reported message for the back end in
question. This function takes one argument—the server
symbol.
Internally, these functions access
back-end-status-string, so the
nnchoke back end will have its error message stored
in nnchoke-status-string.