Handle 404 on RDS deletion

[markdboyd]

Changes proposed in this pull request:

• add logic to return successfully on deletion if database instance was already deleted

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

None, just making deletion idempotent

[JasonTheMain]

Looks good to me