WIP: XML API: Introduce XML Processor.

See also adamziel#44
See also adamziel#43
See also https://github.com/WordPress/wordpress-develop/compare/trunk...sirreal:wordpress-develop:xml-processor-continue?expand=1

src/wp-includes/xml-api/class-wp-xml-malformed-error.php

@@ -0,0 +1,32 @@
+<?php
+
+class WP_XML_Malformed_Error extends ValueError {
+	public $level_of_concern;
+
+	public function __construct( int $level_of_concern, string $message ) {
+		parent::__construct( $message );
+		$this->level_of_concern = $level_of_concern;
+	}
+
+	public function get_level_of_concern(): int {
+		switch ( $this->level_of_concern ) {
+			case WP_XML_Processor::CONCERNED_ABOUT_EVERYTHING:
+				return 'EVERYTHING';
+
+			case WP_XML_Processor::CONCERNED_ABOUT_CONTENT:
+				return 'CONTENT';
+
+			case WP_XML_Processor::CONCERNED_ABOUT_BENIGN_SYNTAX:
+				return 'BENIGN_SYNTAX';
+
+			case WP_XML_Processor::CONCERNED_ABOUT_INVALID_SYNTAX:
+				return 'INVALID_SYNTAX';
+
+			case WP_XML_Processor::CONCERNED_ABOUT_UNRESOLVABLES:
+				return 'UNRESOLVABLE';
+
+			default:
+				throw new ValueError( "Unknown level of concern: {$this->level_of_concern}" );
+		}
+	}
+}