KeywordBear.py: Output appropriate message

Output appropriate message if the language
given in input is not valid/not supported
for KeywordBear

Fixes #1256

bears/general/KeywordBear.py

@@ -18,8 +18,12 @@ def _get_comments(dependency_results):
             not isinstance(annotation_bear_results, list)):
         return
 
-    for result in annotation_bear_results:
-        yield from result.contents.get('comments', [])
+    try:
+        for result in annotation_bear_results:
+            yield from result.contents.get('comments', [])
+    except AttributeError:
+        raise AttributeError(
+            'Given Language in the input is not supported by KeywordBear')
 
 
 def generate_diff(comments, file, filename,