Don't report exception to Airbrake if simply missing some answer values

Closes #107

LGTM given by: @esposito

Squashed commit of the following:

commit 233b1504549019540b2a7e0cfe2d97510d497dd6
Author: Marten Veldthuis <marten@veldthuis.com>
Date:   Tue Nov 11 15:55:57 2014 +0100

    Please rubocop

commit 8e00480d639afd03d5f50ca323ca680e11e2cf2a
Author: Marten Veldthuis <marten@veldthuis.com>
Date:   Tue Nov 11 15:43:14 2014 +0100

    Raise specific exception, catch and set blank score

commit d10fc1210043e24ec5acb876848ae7154e6fabc1
Author: Marten Veldthuis <marten@veldthuis.com>
Date:   Tue Nov 11 14:36:40 2014 +0100

    Add questionnaire arg to ScoreCalculator