string_decoder: Migrate to use internal/errors

PR-URL: https://github.com/nodejs/node/pull/14682
Refs: https://github.com/nodejs/node/issues/11273


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>