ESLintBear: Show errors shown in stderr

In some cases, eslint fails. Because of this, the user can't see any
results, nor does the user see any warning that it failed. We now show
the stderr to the user as a WARNING.

Fixes coala#730

Author: AbdealiLoKo

bears/js/ESLintBear.py

@@ -8,7 +8,8 @@
 
 
 @linter(executable='eslint',
-        use_stdin=True)
+        use_stdin=True,
+        use_stderr=True)
 class ESLintBear:
     """
     Check JavaScript and JSX code for style issues and semantic errors.
@@ -48,10 +49,15 @@ def generate_config(filename, file):
         return '{"extends": "eslint:recommended"}'
 
     def process_output(self, output, filename, file):
-        if not file or not output:
+        if output[1]:
+            self.warn("While running {0}, some issues were found:"
+                      .format(self.__class__.__name__))
+            self.warn(output[1])
+
+        if not file or not output[0]:
             return
 
-        output = json.loads(output)
+        output = json.loads(output[0])
         lines = "".join(file)
 
         assert len(output) == 1