The Download constructor considers it invalid to have a failure
reason if the download isn't in the failed state. Unfortunately,
calling DefaultDownloadIndex.removeAllDownloads when there's a
failed download will change the state without clearing the reason.
If the downloads are then read back from the DefaultDownloadIndex
we end up violating the Download constructor assertion.
This change clears the failed reason for any existing rows in the
invalid state, and also fixes the root cause that allows invalid
rows to enter the table in the first place.
Issue: #6785
PiperOrigin-RevId: 286576242
87865a5cc4