From MYFACES-3053 "user should see not just a cryptic stack trace, but the EL expression that was being evaluated including the part of the EL expression that triggered the problem"
Myfaces utilize TagValueExpression and TagValueExpressionUEL as TagAtrribute-aware wrappers around EL ValueExpression. But this "context" is only .toString() of TagAttribute and that prohibits user-frendly formatting of messages.
Provide TagAttribute instance, create TagAttributeAwareExceptionWrapper that will hold this instance. Clients (mainly ErrorPage) can read attributes of TagAttribute and format it as necessary, for example "EL expression that triggered the problem: " + wrapper.getTagAttribute().getValue()