Provide clearer error notification if dashboards are visited without entering API key

Problem/Motivation

When you visit the dashboards without first providing an API key, you get the message a "URL using bad/illegal format or missing URL".

I think it would be better to just pass on to the user that they still need to enter the API key before the data will be retrieved.

Steps to reproduce

Enable the modules activecampaign and activecampaign_dashboard

Proposed resolution

Provide better error handeling.

Remaining tasks

Add the code, make a MR and merge after testing.

User interface changes

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API changes

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Data model changes

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