For example, the most basic error, cvc-elt-1: you can see the typical kinds of description for it here.  It will give you an error like:

cvc-elt.1: Cannot find the declaration of element 'unobtanium'.

But this is misleading.  Most times I get this error, the problem is not that it cannot find a declaration, that is actually another symptom. The problem is that it does not have a schema with a declaration for that element in that namespace.