Possible Error Message Constants

[abMatillion]

Would it be possible to have consts for the error messages that the api provides?

The errors codes are provided as consts within error_types.go which helps users of the library who don't need to worry about changes to the api, unfortunaletly there are times when the error code is not sufficient as a measure of the specific issue e.g. when hitting the change subscription endpoint there is the possibility of a validation error whereby the message would say something like Subscription hasn't changed.

I'd rather not assert on the message string within my code and would prefer if the library supported message consts or additional types for cases such as this as otherwise I have to keep an eye out for message changes within api versions which could break my code.

Thoughts?