Bucket names are verified to conform with rules published by S3 -
https://docs.aws.amazon.com/AmazonS3/latest/dev/BucketRestrictions.html

fixes #1308

Misused the wrong assertion method previously. `ok` assertions don't
fail when the conditional throws an exception.

Each class of failed bucket name validation shows the offset of the
offending character.

As suggested on the PR review, current set of tests only validate that
the offsets are computed correctly. This test verifies that the message
is generated as expected.