Ensure we always show backend error in automation and script editor

[spacegaier]

Breaking change

Proposed change

I have an automation currently that I cannot save due to some backend failöure (separate issue). However, in that case the frontend only shows an empty toast as the backend is not actually providing a proper error message.

This PR ensures that we at least show the more generic error to inform the user.

Debug screenshot of generic backend response:

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