Add docs about errors in enum

[sobolevn]

It is a common practice to put possible errors in enum (or Sum Type).

So, we can later use them like so:

from enum import Enum, unique

@unique
class APIExceptions(Enum):
      _value: Exception 
      ServerTimeoutError = exceptions.ServerTimeoutError
      OtherError = exceptions.OtherError

def call_api() -> Result[int, APIExceptions]: ... 

We can also try Union and not enum.

[sobolevn]

Or union. Never mind.