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

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

Index: third_party/WebKit/Source/devtools/scripts/chrome_debug_launcher/launch_chrome.js
diff --git a/third_party/WebKit/Source/devtools/scripts/chrome_debug_launcher/launch_chrome.js b/third_party/WebKit/Source/devtools/scripts/chrome_debug_launcher/launch_chrome.js
index b163451f21b4546d6e82a7817ea662df298c1ebb..0a381f24ce1a146ec4fe2158e11c1bf746e17f0e 100644
--- a/third_party/WebKit/Source/devtools/scripts/chrome_debug_launcher/launch_chrome.js
+++ b/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);