multiversion: headers() do not play nice on CLI

When using MWMultiversion from the CLI, we would get a 'can not modify
header information' whenever an error is emitted.  That is a bit
cumbersome so just skip the headers() call whenever we are on CLI.

Change-Id: Ie32e36802c93d0430958a96bacf4a22bca4d16f8
Merge "multiversion: headers() do not play nice on CLI"