MongoDB input does not report TLS certificate errors

[danielnelson]

Bug report

Relevant telegraf.conf:

[[inputs.mongodb]]
  servers = ["$MONGODB_URL"]
  gather_perdb_stats = false
  ## Optional SSL Config
  ssl_ca = "/ssl/mongodb/ca.pem"
  ssl_cert = "/ssl/mongodb/cert.pem"
  ssl_key = "/ssl/mongodb/key.pem"
  insecure_skip_verify = true

System info:

1.6.0

Steps to reproduce:

1. Provide certificate that cannot be loaded
2. telegraf --input-filter mongodb --test

Expected behavior:

Error indicating the certificate could to be loaded.

Actual behavior:

Falls back to non-TLS connection

E! Error in plugin [inputs.mongodb]: Unable to connect to MongoDB, no reachable servers

Additional info:

MongoDB server says:

I -        [conn380] AssertionException handling request, closing client connection: 17189 The server is configured to only allow SSL connections