Incorrect implementation of custom errors for some common error types

[nikitawootten]

Bug Report:

Some errors in errors.go are just instances of errors, and not custom types that implement the error interface.

Tests for errors such as ErrNotFound can not directly be done without searching for error content.

Expected Behavior

errors.As() allows you to test if a GoGM error is a specific type, such as a ErrNotFound, ErrConnection, etc..

Current Behavior

Many errors do not implement the error

Possible Solution

Correctly implement custom errors