Clarify error message in the event a scan does not complete due to IO error

[ewodrich]

If a vuln-scan or malware-scan fail due to IO error, update the error message to make clear the has failed. This is especially pertinent if a scan begins to populate output and then fails, which may be unclear to a user that the scan did not complete.

Ideas for wording: Scan failed. Error: Unable to scan directory at <directory> Scan could not complete. Error: Unable to scan directory at <directory> Scan stopped before completing. Error: Unable to scan directory at <directory>