* 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'.
* guix/ui.scm (program-name, guix-warning-port): New variables.
(warning): New macro.
(guix-main): Parametrize PROGRAM-NAME.
* guix/scripts/build.scm, guix/scripts/download.scm,
guix/scripts/gc.scm, guix/scripts/package.scm: Adjust to use `leave'
and `warning' consistently.
* guix/store.scm (&nix-connection-error): New condition type.
(open-connection): Translate `system-error' during the `connect' call
into `&nix-connection-error'.
* guix/ui.scm (call-with-error-handling): Add case for `nix-connection-error?'.
* guix/scripts/package.scm (guix-package): Move `open-connection' call
within `with-error-handling'.
* guix/scripts/pull.scm (guix-pull): Likewise.
* guix/scripts/download.scm (guix-download): Move body within
`with-error-handling'.
Nikita Karetnikov