Make 'sender' as requirement for the config (fixes #7698)

[fabaff]

Description:

Make 'sender' as requirement for the configuration. Instead of catching the exception by smtplib.

Related issue (if applicable): fixes #7698

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#2686

Example entry for configuration.yaml (if applicable):

notify:
  - platform: smtp
    name: smtp
    server: !secret ha_email_server
    sender: !secret ha_email_sender
    sender_name: 'asdfasdfsadf'
    port: !secret ha_email_port
    starttls: 1
    username: !secret ha_email_username
    password: !secret ha_email
    recipient:
      - test0@fabian-affolter.ch
      - test1@fabian-affolter.ch

Checklist:

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.

[mention-bot]

@fabaff, thanks for your PR! By analyzing the history of the files in this pull request, we identified @partofthething, @pvizeli and @azogue to be potential reviewers.

[azogue]

LGTM