Add more unit tests for error handling to download_warcs()

[amhanson9]

The function catches errors raised by three other functions: API error for getting the WARC metadata, API error for downloading the WARC, and fixity change after download. 2/3 functions raise two different error types. They all use the same code pattern to handle the raised error.

Currently, the only error handling test is for API error for getting the WARC metadata (input is a WARC name that is not in Archive-It). Add tests for the rest if possible. I'm not sure how to cause them to happen mid-function though.