Use enum for error classification

Close #1

MDErrorCategory.php

@@ -0,0 +1,28 @@
+<?PHP
+/**
+ * Represents an error category.
+ *
+ * @author Joshua Ramon Enslin <joshua@museum-digital.de>
+ */
+declare(strict_types = 1);
+
+/**
+ * Represents an error category.
+ */
+enum MDErrorCategory implements JsonSerializable {
+
+    case unknown;
+    case critical;
+    case known;
+
+    /**
+     * Provides the option to serialize as a string during json_encode().
+     *
+     * @return string
+     */
+    public function jsonSerialize():string {
+
+        return $this->name;
+
+    }
+}

MDErrorReporter.php

@@ -9,13 +9,7 @@ declare(strict_types = 1);
  */
 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 @@ final class MDErrorReporter {
             || $exception instanceof MDcouldNotSetPublic
             || $exception instanceof MDInvalidNodaLink
         ) {
-            return self::MD_ERROR_KNOWN;
+            return MDErrorCategory::known;
         }
 
-        return self::MD_ERROR_UNKNOWN;
+        return MDErrorCategory::unknown;
 
     }