ui: Add a 'define-diagnostic' macro.

* guix/ui.scm (define-diagnostic): New macro, which is based on the
  previous version of 'warning'.
  (warning, leave): Redefine using 'define-diagnostic'.
  (report-error): New macro.
  (install-locale): Use 'warning' instead of 'format'.
  (call-with-error-handling): Adjust 'leave'.
* gnu/packages.scm (package-files): Use 'warning' instead of 'format'.
* guix/gnu-maintenance.scm (http-fetch): Use 'warning' and 'leave'.
* guix/scripts/build.scm (derivations-from-package-expressions, guix-build):
  Adjust 'leave'.
* guix/scripts/download.scm (guix-download): Adjust 'leave'.
* guix/scripts/gc.scm (size->number, %options): Adjust 'leave'.
* guix/scripts/package.scm (roll-back, guix-package): Adjust 'leave'.
* po/POTFILES.in: Add 'guix/gnu-maintenance.scm'.