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;
+
+    }
+}