* guix/ui.scm (load*): Add #:on-error parameter.
[tag, error-string]: New variables.
Wrap 'load' call in 'call-with-prompt'. Pass TAG to 'make-stack'. Honor
ON-ERROR after 'report-load-error' call.
(report-load-error): Change to not exit on error. Make private.
* guix/scripts/system.scm (show-help, %options): Add --on-error.
(guix-system): Use 'load*' and pass it #:on-error.