web: Issue more detailed errors for D-Bus disconnections

We want to distinguish between early vs late disconnections.  With the
former, the call wasn't made at all, while with the latter the call
was made but we didn't receive the reply.

Reviewed-by: Stef Walter <stefw@redhat.com>
web: Use dbus-json2 for #server

No changes are necessary

Reviewed-by: Stef Walter <stefw@redhat.com>
web: Use a dialog for shutdown/reboot

Fixes #653
Fixes #50
Closes #656
Reviewed-by: Stef Walter <stefw@redhat.com>