Error collections mentioned in the SRFI so far:

'errno        POSIX errnos
'postgresql   PostgreSQL errors
'libsodium    The libsodium cryptography library

As John mentioned earlier, errno originates from the C language itself,
and is defined in the ISO/ANSI C standard. POSIX tracks that standard
and POSIX errno values are a superset of C errno values. Particular
Unix-like OSes further extend the POSIX set of errno identifiers with
some new ones (and occasionally omit some POSIX ones).

Hence 'errno is a good symbol (accurately mirrors the vagueness of the
underlying C variable as to where each error identifier comes from: C,
POSIX, non-POSIX Unix heritage, or OS-specific). The description could
be something similarly vague like "C/POSIX/Unix errno values".

Arthur, could we keep a growing list of known error-set identifiers in
the SRFI's repo? The list could also go into a file in
<https://github.com/srfi-explorations/identifiers> which hosts the list
of Scheme implementation ID's I've been collecting based on the
cond-expand identifiers used by each implementation.

Here are some more:

'ftp          File Transfer Protocol reply codes
'http         Hypertext Transfer Protocol status codes
'mysql        MySQL and MariaDB database errors
'sqlite       SQLite database result codes

And usage examples:

(foreign-error:error-set ferr) -> 'ftp
(foreign-error:code ferr) -> 504
(foreign-error:message ferr) -> "Command not implemented for that parameter"

(foreign-error:error-set ferr) -> 'http
(foreign-error:code ferr) -> 404
(foreign-error:message ferr) -> "Not Found"

(foreign-error:error-set ferr) -> 'mysql
(foreign-error:code ferr) -> 1005
(foreign-error:symbol ferr) -> 'ER_CANT_CREATE_TABLE
(foreign-error:message ferr) -> "Can't create table '%s' (errno: %d)"

(foreign-error:error-set ferr) -> 'sqlite
(foreign-error:code ferr) -> 6
(foreign-error:symbol ferr) -> 'SQLITE_LOCKED
(foreign-error:message ferr) -> "A table in the database is locked"