Move error_printf_unless_qmp() with monitor unit

Since it depends on monitor code, and error_vprintf_unless_qmp() is
already there.

This will help to move error-report in a common subproject.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220420132624.2439741-31-marcandre.lureau@redhat.com>