DevTools: Don't show an error in npm start when CHROMIUM_PATH is missing

BUG=none

Review-Url: https://codereview.chromium.org/2472913005
Cr-Commit-Position: refs/heads/master@{#430136}

third_party/WebKit/Source/devtools/scripts/chrome_debug_launcher/launch_chrome.js

@@ -78,6 +78,9 @@ function launchChrome(filePath, chromeArgs, cleanup)
         });
     } catch (error) {
         onLaunchChromeError();
+        if (cleanup)
+            cleanup();
+        return;
     }
     child.on("error", onLaunchChromeError);
     child.on("exit", onExit);