BF: Use error code 501 NotImplemented
BF: Use error code 501 NotImplemented

Fix #551
Pass `errors.NotImplemented` (err code 501)
instead of `errors.InternalError` (err code 500)

lucisilv/Silviu_S3-Antidote,tomateos/S3,tomateos/S3,tomateos/S3,scality/S3,scality/S3,scality/S3,truststamp/S3,scality/S3,truststamp/S3,tomateos/S3,scality/S3,scality/S3,lucisilv/Silviu_S3-Antidote,tomateos/S3,lucisilv/Silviu_S3-Antidote,tomateos/S3,truststamp/S3,scality/S3,lucisilv/Silviu_S3-Antidote,tomateos/S3,lucisilv/Silviu_S3-Antidote,lucisilv/Silviu_S3-Antidote,truststamp/S3,scality/S3,lucisilv/Silviu_S3-Antidote,truststamp/S3,truststamp/S3,truststamp/S3,truststamp/S3,lucisilv/Silviu_S3-Antidote,tomateos/S3