Validation failing with "invalid xml" for incorrect file path

Describe the Bug

When providing the wrong path to a file, the error given is invalid xml (status=WITH_ERRORS). Nothing in the error points to the real problem: the path provided is not a file.

Steps to Reproduce

const validator = require('xsd-schema-validator');
const schemaFile = './path/to/schema.xsd';
const file = './foo.xml'; // Wrong path - file does not exist
validator.validateXML({ file }, schemaFile, function(err, result) {
   if (err) throw err;
});

Expected Behavior

Error should give some indication that the file you're trying to test is not actually where you think it was.

Environment

• Node v14.16.0
• OS: Windows
• Version 0.7.0

Start at the validateXML entry point shown in the reproduction and trace how the provided file path is handled. Add coverage for the nonexistent-path case and verify that the resulting error identifies the missing file rather than reporting only invalid XML.