Fix test failure message when headless out isn't json

Summary: Jest expects any promise rejections to be errors, not strings.

Reviewed By: passy

Differential Revision: D16200876

fbshipit-source-id: 7ebb6c63262b3c13ec677b1e72010146aa5b55e8

headless-tests/__tests__/headlessIntegrationTests.js

@@ -68,7 +68,9 @@ const runHeadless = memoize((args: Array<string>) => {
       } catch (e) {
         console.warn(stderrChunks.join(''));
         reject(
+          new Error(
             `Failed to parse headless output as JSON (${e.message}): ${stdout}`,
+          ),
         );
       }
     });