server: make sure exc_info is cleared in the callback

--HG--
extra : transplant_source : %5B3%A0%92%C3%CA%AC%8E%DFz%9C%40%D7%9A%81%CCC%84%9A%D6

gevent/server.py

@@ -132,6 +132,7 @@ class StreamServer(BaseServer):
         except:
             traceback.print_exc()
             ex = sys.exc_info()[1]
+            sys.exc_clear()
             if self.is_fatal_error(ex):
                 self.kill()
                 sys.stderr.write('ERROR: %s failed with %s\n' % (self, str(ex) or repr(ex)))