Use enum for error classification

Close #1

MDErrorReporter.php

@@ -9,13 +9,7 @@
  */
 final class MDErrorReporter {
 
-    const MD_ERROR_UNKNOWN  = 20;
-    const MD_ERROR_CRITICAL = 10;
-    const MD_ERROR_KNOWN    = 0;
-
-    /** @var string */
     private string $_context;
-    /** @var string */
     private string $_contact_mail_addr;
 
     /**
@@ -23,9 +17,9 @@ final class MDErrorReporter {
      *
      * @param Throwable $exception Exception to be categorized.
      *
-     * @return integer
+     * @return MDErrorCategory
      */
-    public static function categorizeError(Throwable $exception):int {
+    public static function categorizeError(Throwable $exception):MDErrorCategory {
 
         if ($exception instanceof MDpageParameterMissingException
             || $exception instanceof MDpageParameterNotNumericException
@@ -44,10 +38,10 @@ public static function categorizeError(Throwable $exception):int {
             || $exception instanceof MDcouldNotSetPublic
             || $exception instanceof MDInvalidNodaLink
         ) {
-            return self::MD_ERROR_KNOWN;
+            return MDErrorCategory::known;
         }
 
-        return self::MD_ERROR_UNKNOWN;
+        return MDErrorCategory::unknown;
 
     }