bug#63115: match-record cryptic error message on unbound record type

[Maxim Cournoyer]

Hi,

When a record type is not in scope, the error message produced by the
match-record macro from (guix records) reads something like:

--8<---------------cut here---------------start------------->8---
guix/records.scm:598:32: map-fields: bad use of syntactic keyword in subform 
map-fields of map-fields
--8<---------------cut here---------------end--------------->8---

It'd be nicer if it could hint the developer what the error is exactly
(unknown record type).

Reproducer:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)>

(use-modules (gnu system accounts)
             (guix records))

(define dummy-account
  (user-account (name "dum") (group "dum")))

(match-record dummy-account <user-account>
  (name)
  name)

While compiling expression:
Syntax error:
guix/records.scm:598:32: map-fields: bad use of syntactic keyword in
subform map-fields of map-fields
--8<---------------cut here---------------end--------------->8---

This was caused by <user-account> not being exported from (gnu system
accounts).

-- 
Thanks,
Maxim