services: herd: Move UI handling to 'guix system'.

This makes (gnu services herd) independent of (guix ui).

* gnu/services/herd.scm (&shepherd-error, &service-not-found-error)
(&action-not-found-error, &action-exception-error)
(&unknown-shepherd-error): New error condition types.
(report-action-error): Remove.
(raise-shepherd-error): New procedure.
(display-message): Do not use 'info' and '_'.
(invoke-action): Use 'raise-shepherd-error' instead of
'report-action-error'.  Do not use 'warning'.
(current-services): Do not use 'warning'.
* guix/scripts/system.scm (with-shepherd-error-handling): New macro.
(report-shepherd-error, call-with-service-upgrade-info): New
procedures.
(upgrade-shepherd-services): Use it.

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