Support for converting rlclient lib error codes to string descriptions

[rajan-chari]

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Currently all API methods take an apistatus object if error details are needed as strings. It would be good to have a utility method to convert the error codes into strings.

[lokitoth]

One way to do something like this is to set up a single "ground truth" source of constants (config keys/values, error numbers, error strings) to be shared between the different bindings and RLClientLib. Then rather than opening up an API, we simply generate the language-appropriate constants code.

[jackgerrits]

We actually currently have this and it is how I generated the relevant exceptions in Python and error constants in C++.

See this file: https://github.com/VowpalWabbit/reinforcement_learning/blob/master/include/errors_data.h