SMTP internals displayed in UI on SMTP connection error

[pboguslawski]

When non-existing DNS SMTP server name is configured in user/config/plugins/email.yaml i.e.

mailer:
  engine: smtp
  smtp:
    server: nonexisting-server
    port: 25
    encryption: none
    user: null
    password: null
    auth_mode: null
  sendmail:
    bin: '/usr/sbin/sendmail -bs'

and contact form is defined like this

form:
    name: contact-us
    fields:
        name:
            type: text
            label: 'Name'
            placeholder: 'Give name.'
            autocomplete: 'on'
            validate:
                required: true
            maxlength: 255
[...]
buttons:
        submit:
            type: submit
            value: Submit
    process:
        timestamp:
            label: 'Time'
        ip:
            label: 'IP'
        email:
            to:
                - test@example.com
            subject: 'Message from {{ form.value.name|e }}'
            body: '{% include ''forms/data.html.twig'' %}'
        redirect: /thankyoupage

and user/config/system.yaml contains

errors:
  display: 0
  log: true

or

errors:
  display: -1
  log: true

or

errors:
  display: -1
  log: false

then after pressing Submit Grav displays an error in UI with connection error details

Connection could not be established with host "nonexisting-server:25": stream_socket_client(): php_network_getaddresses: getaddrinfo failed: Temporary failure in name resolution

According to

https://learn.getgrav.org/17/basics/grav-configuration#errors

such internal details should not be displayed in UI when

errors:
  display: 0
  log: true

is set in system.yaml.