Improve error handling on invalid API key

[mvaaltola]

Handle invalid API keys gracefully and show the user an error message.
Fixes #19

Testing

• Open the plugin for the first time. The settings menu opens. Input an invalid API key:

• Hit OK. The following error message is shown and the user is returned to the settings menu:

• Input a wrong API key again. Same error message as above.

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• Re-open the plugin. Input a correct API key. Open the settings menu, change the API key to invalid, hit OK.
• Try to download data. The following error message is shown:

• Re-open the plugin. The API key error message is shown and the Settings menu opens