Download cache: Improve logs on erroneous cases

[gregory-paidis-sonarsource]

DownloadEphemeralUrl fetches a json, which looks like:

• { Url: "...", enabled: true } on cache-hit.
• { enabled: false } on cache-miss.

Right now, we only handle the success scenario and always try to access jsonData.Uri, which results in a NullReferenceException on the sad-path and the log message is not very friendly:

12:04:06.76  12:04:06.76  WARNING: Incremental PR analysis: an error occurred while retrieving the cache entries! Value cannot be null.
Parameter name: uriString

We should handle the sad-path and make the logs more professional and user-friendly.