Log exception tracebacks when they're available for error-messages

See ticket #100

test/test_logging.py

@@ -141,6 +141,17 @@ def test_info(handler, framework):
     assert handler.messages[0].endswith(b"hilarious elephant")
 
 
+def test_legacy_error_with_traceback(handler, framework):
+    import logging
+
+    try:
+        raise RuntimeError("the bad stuff")
+    except Exception:
+        logging.error("bad stuff", exc_info=True)
+
+    assert 'RuntimeError: the bad stuff' in str(handler.messages)
+
+
 def test_trace(handler, framework):
     logger = txaio.make_logger()
     old_log = txaio.get_global_log_level()

txaio/aio.py

@@ -209,6 +209,10 @@ class _TxaioFileHandler(logging.Handler, object):
             dt = datetime.fromtimestamp(record.args.get('log_time', 0))
         else:
             message = record.getMessage()
+            if record.levelno == logging.ERROR and record.exc_info:
+                message += '\n'
+                for line in traceback.format_exception(*record.exc_info):
+                    message = message + line
             dt = datetime.fromtimestamp(record.created)
         msg = u'{0} {1}{2}'.format(
             dt.strftime("%Y-%m-%dT%H:%M:%S%z"),