Resolve Specific Error Codes into Meaningful Messages

This adds global `ToString(<error enum>)` functions to some selected error code enumerations. The idea is, to simply call `ToString(retval)` to retrieve a meaningful string based on the enum-type. I consider this to be a first step since I certainly didn't find all possible places in the code to use this mappings.

*Note:* The return values were not always type-safe (assigning enum-type to central `int retval`). Also the mappings need to be kept in sync with newly added return codes.

@Jakob: Please check, if the error messages in the mappings make sense for the return codes. I wasn't sure in all cases.