JAXP RI's XML Schema validator silently ignores <xs:import>s that failed to
resolve for some reason — be it unreachable resource, a well-formedness error
in the referenced XML, etc.
The only failure reported is when one actually tries to refer to those components.
While I know this is still within the conformance of XML Schema spec, in
practice this makes error diagnostics hard, since people have no clue why their
seemingly valid schema reference is not taking any effect.
A warning about the failed import would be highly desirable.