Clean up source location message creation.
Clean up source location message creation.

pfmoore/pip,pypa/pip,xavfernandez/pip,rouge8/pip,pradyunsg/pip,pypa/pip,rouge8/pip,sbidoul/pip,pfmoore/pip,rouge8/pip,pradyunsg/pip,xavfernandez/pip,sbidoul/pip,xavfernandez/pip