feat: add NewError function

[dzakaammar]

Since every errors from each API will be wrapped by the Error type. Here i add a new function to initialize the Error type publicly. Typically, this function will be useful for unit testing and mocking purposes in the client side to test their behavior by the API response.

[dzakaammar]

hi @nukosuke kindly review this PR ya. thank you :)

[nukosuke]

this function will be useful for unit testing and mocking purposes in the client side to test their behavior by the API response.

Might want to add this commentary to doc comment to explain why the constructor of API error is exposed.

[dzakaammar]

@nukosuke sure, I just update the comment. thank you

[dzakaammar]

thanks @nukosuke, it would be nice if you could create tag for it :)