Handle HTTP errors from the openHAB REST API

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Sometimes, the openHAB REST API throws errors, those are currently not catched / handled by this application. The most likely error to handle would be 503 which is thrown when openHAB is overloaded handling the requests (which could easily happen if computing-intensive actions are triggered via the rotation wheel when not using sendFrequency).

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The basic implementation is done, now an error icon is shown if any exception occurs during event handling. Special topics:

• schedule auto-retry after an HTTP 503 error occurred
• show specific icons on standard cases (e.g.: TOGGLE on NULL state)
• ...?