mgr/telegraf: catch FileNotFoundError exception

mgr/telegraf: catch FileNotFoundError exception

in tasks/module_selftest.yaml, `TestModuleSelftest.test_telegraf()` is
called. but we fail to prepare a unix domain socket to which the telegraf
module can send stats. and telegraf module does not catch
FileNotFoundError exception, so the exception is populated to ceph-mgr
and is found by the test, hence the test is marked a failure whenever
telegraf is tested.

in this change,

* catch this exception, so it won't be caught by ceph-mgr
* whitelist the error message, so the test can pass

Signed-off-by: Kefu Chai <kchai@redhat.com>