Let logging module handle traceback printing

Manually including the traceback in the log msg causes some issues when
interacting with log formats, and in one case was causing the traceback
to be printed twice.

gunicorn/arbiter.py

@@ -207,8 +207,8 @@ def run(self):
         except SystemExit:
             raise
         except Exception:
-            self.log.info("Unhandled exception in main loop:\n%s",
-                          traceback.format_exc())
+            self.log.info("Unhandled exception in main loop",
+                          exc_info=1)
             self.stop(False)
             if self.pidfile is not None:
                 self.pidfile.unlink()
@@ -518,14 +518,13 @@ def spawn_worker(self):
         except SystemExit:
             raise
         except AppImportError as e:
-            self.log.debug("Exception while loading the application: \n%s",
-                           traceback.format_exc())
+            self.log.debug("Exception while loading the application",
+                           exc_info=1)
             print("%s" % e, file=sys.stderr)
             sys.stderr.flush()
             sys.exit(self.APP_LOAD_ERROR)
         except:
-            self.log.exception("Exception in worker process:\n%s",
-                               traceback.format_exc())
+            self.log.exception("Exception in worker process"),
             if not worker.booted:
                 sys.exit(self.WORKER_BOOT_ERROR)
             sys.exit(-1)