Create named constants for error codes in bindings

[lokitoth]

Right now to make use of error codes in RL.Net's RLException we need to use magic numbers based on the C++-side header file.

It would be good to have the same constants (or an enum, with translation inside of RL.Net).

Maybe something like https://github.com/SunsetQuest/CppHeaderToCSharpConverter can be used.

#45 as a related issue.