update delete logic to handle missing bucket

[markdboyd]

Changes proposed in this pull request:

Related to #84

While trying to delete an S3 service, somehow the broker has gotten into a state where the bucket is already deleted, but the service still exists in CF. In this state, subsequent delete-service attempts fail because the deleteBucketContents function does not handle the case where the bucket does not exist. If the bucket does not exist, the deleteBucketContents should not an error and just continue

• update delete objects logic to handle case where bucket does not exist
• update unit tests

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

No expected security changes, just hopefully resolving a broker bug