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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.