[eslint] Don't treat eslint warnings as PRESUBMIT errors

catapult_build/js_checks.py

Index: catapult_build/js_checks.py
diff --git a/catapult_build/js_checks.py b/catapult_build/js_checks.py
index 5c6b68716070e81ad1ba0ba3f7385cc76b1b559d..0efbf0d6ab4b0c78797745e4b2c845d7b34646af 100644
--- a/catapult_build/js_checks.py
+++ b/catapult_build/js_checks.py
@@ -84,10 +84,10 @@ class JSChecker(object):
         error_lines += filter(None, [self.ConstCheck(i, line)])
 
     if affected_js_files:
-      eslint_output = eslint.RunEslintOnFiles(
-          [f.AbsoluteLocalPath() for f in affected_js_files]).rstrip()
+      eslint_output, were_errors = eslint.RunEslint(
+          [f.AbsoluteLocalPath() for f in affected_js_files])
 
-      if eslint_output:
+      if were_errors:
         error_lines.append('\neslint found lint errors:')
         error_lines.append(eslint_output)