Truncated error message in cache's self-check feature

[jhiemstrawisc]

We've observed unintended error message truncation in this function. The error message printed out wasself-test failed during download. File is cleaned up at without the file URL.

Let's make sure this truncation can't happen by making sure fileUrl can't be the empty. This probably also indicates there was an uncaught error in generateTestFile().

On that note -- change all the references from "files" to "objects". Eventually we'll be able to back caches with non-posix object stores.