Refactor logging handling

Old system was using manual string formatting for log messages.
This caused issues with common operations like exception logging
because often they need to be handled differently for Py2/Py3
compatibility. In order to unify the effort:

 - All logging is changed to `logging.level(msg, arg1, arg2)` style.
 - A `SafeLogger` is implemented to auto-decode exceptions properly
in the args (ref #1403).
 - Custom formatters were overriding useful logging functionality
like traceback outputing (ref #1402). They are refactored to be
more transparent. Traceback information is provided in `--debug`
mode for `read_file` errors in generators.
 - Formatters will now auto-format multiline log messages in order
to make them look related. Similarly, traceback will be formatted in
the same fashion.
 - `pelican.log.LimitFilter` was (ab)using logging message which
would result in awkward syntax for argumented logging style. This
functionality is moved to `extra` keyword argument.
 - Levels for errors that would result skipping a file (`read_file`)
changed from `warning` to `error` in order to make them stand out
among other logs.
 - Small consistency changes to log messages (i.e. changing all
to start with an uppercase letter) and quality-of-life improvements
(some log messages were dumping raw object information).

pelican/settings.py

@@ -152,7 +152,8 @@ def read_settings(path=None, override=None):
             local_settings['PLUGIN_PATHS'] = local_settings['PLUGIN_PATH']
             del local_settings['PLUGIN_PATH']
         if isinstance(local_settings['PLUGIN_PATHS'], six.string_types):
-            logger.warning("Defining %s setting as string has been deprecated (should be a list)" % 'PLUGIN_PATHS')
+            logger.warning("Defining PLUGIN_PATHS setting as string "
+                           "has been deprecated (should be a list)")
             local_settings['PLUGIN_PATHS'] = [local_settings['PLUGIN_PATHS']]
         elif local_settings['PLUGIN_PATHS'] is not None:
                 local_settings['PLUGIN_PATHS'] = [os.path.abspath(os.path.normpath(os.path.join(os.path.dirname(path), pluginpath)))
@@ -201,7 +202,7 @@ def configure_settings(settings):
                         ' (see pelican --help for more information)')
 
     # specify the log messages to be ignored
-    LimitFilter.ignore.update(set(settings.get('LOG_FILTER',
+    LimitFilter._ignore.update(set(settings.get('LOG_FILTER',
                                                DEFAULT_CONFIG['LOG_FILTER'])))
 
     # lookup the theme in "pelican/themes" if the given one doesn't exist
@@ -243,10 +244,9 @@ def configure_settings(settings):
             ]:
         if key in settings and not isinstance(settings[key], types):
             value = settings.pop(key)
-            logger.warn(
-                'Detected misconfigured {} ({}), '
-                'falling back to the default ({})'.format(
-                    key, value, DEFAULT_CONFIG[key]))
+            logger.warn('Detected misconfigured %s (%s), '
+                        'falling back to the default (%s)',
+                    key, value, DEFAULT_CONFIG[key])
 
     # try to set the different locales, fallback on the default.
     locales = settings.get('LOCALE', DEFAULT_CONFIG['LOCALE'])
@@ -319,8 +319,8 @@ def configure_settings(settings):
         old_key = key + '_DIR'
         new_key = key + '_PATHS'
         if old_key in settings:
-            logger.warning('Deprecated setting {}, moving it to {} list'.format(
-                old_key, new_key))
+            logger.warning('Deprecated setting %s, moving it to %s list',
+                old_key, new_key)
             settings[new_key] = [settings[old_key]]   # also make a list
             del settings[old_key]
 
@@ -343,8 +343,8 @@ def configure_settings(settings):
     for PATH_KEY in filter(lambda k: k in settings, path_keys):
         if isinstance(settings[PATH_KEY], six.string_types):
             logger.warning("Detected misconfiguration with %s setting "
-                           "(must be a list), falling back to the default"
-                           % PATH_KEY)
+                           "(must be a list), falling back to the default",
+                           PATH_KEY)
             settings[PATH_KEY] = DEFAULT_CONFIG[PATH_KEY]
 
     # Add {PAGE,ARTICLE}_PATHS to {ARTICLE,PAGE}_EXCLUDES