Initial unit tests, code cleanup

src/UnknownDatasetExeption.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace NoccyLabs\Dataset;
+
+use RuntimeException;
+
+class UnknownDatasetException extends RuntimeException
+{
+    public static function DatasetNotFound(): self
+    {
+        return new UnknownDatasetException("The requested dataset is not available");
+    }
+}