Setting select-context=key on the schematron xsl.

This causes it to output better xslt so that we don't
get those error messages. In the xslt there's some doc
as follows (note that "select-contents" is a typo for
"select-contexts"):

 Experimental: USE AT YOUR OWN RISK
    visit-text "true" "false"   Also visist text nodes for context. WARNING: NON_STARDARD.
    select-contents '' | 'key' | '//'   Select different implementation strategies

So I'm not sure if setting the param breaks anything.