Support smaller error indication

[MarcelGarus]

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Currently, we show errors by displaying a broken pen. At several places, like on the dashboard, in course cards, in the account avatar, etc. this just takes too much place. We should think about a generic way how to show errors in such situations.

[MarcelGarus]

#334 introduces some error handling for the cases from above. I think we shouldn't provide a generic way of handling errors, but rather think about error handling in those cases separately and about how to display errors in the given context in the best way.