When you run the following code,

        SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI)
            .newSchema(new File("No such schema"));

You get the following error:

org.xml.sax.SAXParseException: schema_reference.4: Failed to read schema document 'file:///c:/kohsuke/Sun/jaxp/jaxp-sources/No such schema', because 1) could not find the document; 2) the document could not be read; 3) the root element of the document is not <xsd:schema>.

This is embarassingly silly.

- case 1,2 and case 3 are easily distinguishable. so please use two different error messages.

- give us the nested exception that shows the underlying cause. That can tell me whether
  the resource was not found, or whether there was an IO error.

- in case 3, please give us the name of the element you found.