Ensure more consistent lovelace config errors

[spacegaier]

Breaking change

Proposed change

1. Added proper errors where entity is required, rather than not having an empty, but still space using card
2. Check array length where multiple entities can be specified (a simple check for existance or is-array not sufficient since during initial editor creation all have an entity set, but user can remove it)
3. Mark entity input fields as required (where previously incorrectly marked as optional)
4. Consistent wording of errors

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

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

• Documentation added/updated for www.home-assistant.io