Update error page

[AlanGreene]

Update and unify our current error handling to display more useful content to the user in case they hit a bad link, or try to access some feature or resource to which they do not have access.

This will be particularly useful when we make more progress on #1420 and related issues.

[kamilzal]

Final outcome

Assets: robocat_404.svg.zip

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Illustration size @ 1312px width

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Text and Spacing

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Alignment

1. Align text to columns.
2. Align illustration to start at the middle of gutter between 2 columns.
3. Grouping of illustration + message should be centered inside the content area.