devserver: Fix linter errors

Also remove devserver_exception.py as there was no user except devserver
itself.

BUG=chromium:1003986
TEST=./devserver_integration_test.py

Change-Id: Idf5289371bbe17c57fbbdb5de445e083281bd62c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1838133
Tested-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/health_checker.py b/health_checker.py
index 8c274a3..20a56c0 100644
--- a/health_checker.py
+++ b/health_checker.py
@@ -14,7 +14,7 @@
 import threading
 import time
 
-import cherrypy
+import cherrypy  # pylint: disable=import-error
 
 import cros_update_progress
 import log_util