(foreign-error:set ferr) -> symbol or #f  ; error-set
(foreign-error:code ferr) -> exact integer or symbol or #f
(foreign-error:mnemonic ferr) -> symbol or #f
(foreign-error:message ferr [language [country]]) -> string or #f
(foreign-error:message-locales ferr) -> list of (language . country) pairs
(foreign-error:data ferr [key [args...]]) -> object or #f
(foreign-error:data-keys ferr) -> list of symbols
(foreign-error:call ferr) -> (symbol . arguments) or #f          ;
Scheme side
(foreign-error:foreign-call ferr) -> (symbol . arguments) or #f  ;
C/network side

Suggested data keys for things that are common but not ubiquitous:

'facility               symbol
'severity               symbol
'message-detail         string
'message-hint           string
'class-code             symbol or exact integer
'class-title            string
'filename               string
'line-number            positive exact integer
'column-number          positive exact integer
'sqlstate               symbol (or is string better?)