lint-all: Fix progress bar from `npm run lint` in linter.

Also fixes printing the `eslint` help output in the event that there
are no files to check.

See https://github.com/zulip/zulip/pull/2246#issuecomment-261673662
for details on the original problem.

tools/lint-all

@@ -495,7 +495,9 @@ def run():
         @lint
         def eslint():
             # type: () -> int
-            result = subprocess.call(['npm', 'run', '--silent', 'lint']
+            if len(by_lang['js']) == 0:
+                return 0
+            result = subprocess.call(['tools/node', 'node_modules/.bin/eslint', '--quiet']
                                      + by_lang['js'])
             return result