In testing the docker mongodb/atlas container doesn't throw an exception when
calling `mongoTemplate.executeCommand({"createSearchIndexes": ...});`. However,
when using the Atlas service it does and throws a `IndexAlreadyExists` exception.
Added error handling for error code 68 or error code name `IndexAlreadyExists`.
See: https://www.mongodb.com/docs/manual/reference/error-codes/
Fixes #910