records: Improve reporting of "invalid field specifier" errors.

records: Improve reporting of "invalid field specifier" errors.

Previously users would just see:

  error: invalid field specifier

without source location or hints.

* guix/records.scm (expand): Add optional 'parent-form' parameter and
pass it to 'syntax-violation' when it is true.
(make-syntactic-constructor): Pass S as a third argument to
'report-invalid-field-specifier'.
* guix/ui.scm (report-load-error): For 'syntax-error', show SUBFORM or
FORM in the message.
* tests/records.scm ("define-record-type* & wrong field specifier"): Add
a 'subform' parameter and adjust test accordingly.
("define-record-type* & wrong field specifier, identifier"): New test.
* tests/guix-system.sh: Add test.