[core] add custom error messages for stdlib_ex, and a test

core/tests/StdLibExTest.php

@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Shimmie2;
+
+class StdLibExTest extends ShimmiePHPUnitTestCase
+{
+    public function testJsonEncodeOk(): void
+    {
+        $this->assertEquals(
+            '{"a":1,"b":2,"c":3,"d":4,"e":5}',
+            json_encode_ex(['a' => 1, 'b' => 2, 'c' => 3, 'd' => 4, 'e' => 5])
+        );
+    }
+
+    public function testJsonEncodeError(): void
+    {
+        $e = $this->assertException(\Exception::class, function() {
+            json_encode_ex("\xB1\x31");
+        });
+        $this->assertEquals(
+            "Malformed UTF-8 characters, possibly incorrectly encoded",
+            $e->getMessage()
+        );
+    }
+}