Replace AttErrorCode enum with newtype wrapping u8

[petekubiak]

          I think we should consider making this a newtype wrapping `u8` with associated constants, rather than an enum. When a client sends a request to a remote server that server can theoretically return any error code it wants, regardless of the spec. For example, if it's a server that supports a newer version of the spec than trouble it could return a code unknown to trouble.

Originally posted by @alexmoon in #151 (comment)