Raising exception instead of printing error on bad YAML

bin/util.py

@@ -68,8 +68,7 @@ def read_markdown(parser, path):
         try:
             metadata = yaml.load(pieces[1])
         except yaml.YAMLError as e:
-            print('Unable to parse YAML header in {0}:\n{1}'.format(path, e))
-            sys.exit(1)
+            raise ValueError('Unable to parse YAML header in {0}:\n{1}'.format(path, e))
         metadata_len = pieces[1].count('\n')
         body = pieces[2]