logging: Add exc() and exception() methods.

Non-standard exc() method accepts exception instance to log as a
parameter. exception() just uses sys.exc_info().

logging/logging.py

@@ -59,6 +59,13 @@ class Logger:
     def critical(self, msg, *args):
         self.log(CRITICAL, msg, *args)
 
+    def exc(self, e, msg, *args):
+        self.log(ERROR, msg, *args)
+        sys.print_exception(e, _stream)
+
+    def exception(self, msg, *args):
+        self.exc(sys.exc_info()[1], msg, *args)
+
 
 _level = INFO
 _loggers = {}