Guard setting up config flow for an unsupported domain

[balloob]

Breaking change

Proposed change

@ludeeus figured out why we say error messages with [object Object]. It's reproduced pointing a My link at a domain that doesn't support config flows. It's a rest endpoint and so the thrown error follows a different format than if a WebSocket endpoint fails.

I've added an extra guard to the My link to ensure that we check the config flow exists before we ask the user if they want to set it up.

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

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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