API: Return a 404 error when no director rule is matched

control/Director.php

@@ -331,6 +331,9 @@ class Director implements TemplateGlobalProvider {
 				}
 			}
 		}
+
+		// No URL rules matched, so return a 404 error.
+		return new SS_HTTPResponse('No URL rule was matched', 404);
 	}
 	
 	/**

tests/control/DirectorTest.php

@@ -281,6 +281,10 @@ class DirectorTest extends SapphireTest {
 		$this->assertEquals($headers, Director::extract_request_headers($request));
 	}
 
+	public function testUnmatchedRequestReturns404() {
+		$this->assertEquals(404, Director::test('no-route')->getStatusCode());
+	}
+
 }
 
 class DirectorTestRequest_Controller extends Controller implements TestOnly {