EXECUTE: Create JSF 2 KickStart project.
EXECUTE: Open WebContent/resources/demo/input.xhtml
EXECUTE: Add

<h:outputText>#{msgs.error}</h:outputText>

to <h:form>:

<composite:implementation>
    	<h:form>
    		<h:outputText value="#{cc.attrs.label}" />
    		<h:inputText value="#{cc.attrs.value}" />
    		<h:commandButton action="#{cc.attrs.action}" value="#{cc.attrs.submitlabel}" />
    		<h:outputText>#{msgs.error}</h:outputText>
    	</h:form>
</composite:implementation>

EXECUTE: Save the file.
FAILURE: '"error" cannot be resolved' EL Knowledge Base Problem does not appear
ASSERT: '"error" cannot be resolved' EL Knowledge Base Problem appears

BTW validation for attributes works fine:
<h:outputText value="#

"/>

The problem is in PageContextFactory which returns the context w/o any ELReference for this page.