Adds AV_STATUS_FAILED configuration variable to be set as the status
when a file cannot be scanned. Previously files that failed scanning
would never be tagged, which means they would be picked up in every
subsequent re-scan as well as ambiguous in status for users.

Files may fail to scan if they are too large (due to either being larger
than lambda's /tmp, or just large enough that clamscan fails when trying
to mmap the signature databases). This commit cleans up the
lambda_handler a bit and handles OSError from the s3 download running
out of disk space.