Adds simple handler to provide failed line numbers

Change adds `as err` handler to provide line number, if config yaml
fails to parse.

Example output

[config]  ERROR   while scanning a simple key
  in "config.yaml", line 5, column 1
could not find expected ':'
  in "config.yaml", line 6, column 1
[main]  ERROR   config.yaml : Error parsing file.

Change-Id: If764a123c0dd8871dcd98f58be48c6bf0034f1d4
Closes-Bug: #1621552

bandit/core/config.py

@@ -49,7 +49,8 @@ class BanditConfig(object):
             try:
                 self._config = yaml.safe_load(f)
                 self.validate(config_file)
-            except yaml.YAMLError:
+            except yaml.YAMLError as err:
+                LOG.error(err)
                 raise utils.ConfigError("Error parsing file.", config_file)
 
             # valid config must be a dict