Previously field type errors would be reported in a non-standard way,
and without any source location information. This fixes it.
* gnu/services/configuration.scm (configuration-field-error): Add a
'loc' parameter and honor it. Use 'formatted-message' instead of plain
'format'.
(define-configuration-helper)[field-sanitizer]: New procedure.
Use it. Use STEM as the identifier of the syntactic constructor of the
record type. Add a 'sanitize' property to each field. Remove now
useless STEM macro that would call 'validate-configuration'.
* gnu/services/mail.scm (serialize-listener-configuration): Adjust to
new 'configuration-field-error' prototype.
* tests/services/configuration.scm ("wrong type for a field"): New test.
* po/guix/POTFILES.in: Add gnu/services/configuration.scm.
