Modified AWS S3 mock, so that it errors when creating an existing bucket

AWS S3 returns a BucketAlreadyOwnedByYou error when attempting to create a bucket that already exists, in a region other than the default. This commit modifies the mocked object to reflect this.
Merge pull request #2461 from GiMiScale/fix-mock-s3-create-bucket

Modified AWS S3 mock, so that it errors when creating an existing bucket