Improve Error Handling

[SchulzeStTSI]

It was reported that the error handling for the REST Client is not ideal solved or document. The process how errors are highlighted must be improved to create an proper exception handling outside of the dgc lib. (e.g. for connection errors, verification errors etc.)

[nodh]

I'm upvoting this issue. Appearantly, if an error on downloading trusted certificates occurs, the error is simply logged. The caller (i.e. the application using this lib) does not get any information about the error, besides an empty collection of trusted certificates.

[JesusMcCloud]

Upvoting as well. When an error occurs, while instantiating a DgcGatewayRevocationListDownloadIterator this error is simply logged and it behaves as if no data is present and everything's fine.