This allows 'gettext' to be called on the format string at the site
where the exception is caught (rather than the site where it's thrown).
It also allows for argument highlighting.
* guix/diagnostics.scm (&formatted-message): New condition type.
(check-format-string): New procedure.
(formatted-message): New macro.
* guix/ui.scm (report-load-error): Add clause for 'formatted-message?'.
(warn-about-load-error): Likewise.
(call-with-error-handling): Likewise.
(read/eval): Likewise.
Callers can pass 'report-error', 'warning', etc. to 'apply'.
* guix/diagnostics.scm (trivial-format-string?): New procedure, moved
from...
(highlight-argument): ... here.
(define-diagnostic): Add 'identifier?' clause.
(emit-diagnostic): New procedure.
