This change adds `SessionError` and uses it in `SessionResult`
and `LibraryResult` to report errors to callers.
Constructors and factory method that used a simple `errorCode` to
construct error variants of `SessionResult` and `LibraryResult`
have been overloaded with a variant that uses a `SessionError`
instead. While these methods and constructors are supposed to be
deprecated, they aren't yet deprecated until the newly added
alternative is stabilized.
PiperOrigin-RevId: 642254336
efff1ee2f1