Missing files should not write to disk

[stephen-soltesz]

When a source file does not exist in GCS, the file content is replaced with:

<?xml version='1.0' encoding='UTF-8'?><Error><Code>AccessDenied</Code><Message>Access denied.</Message><Details>Anonymous caller does not have storage.objects.get access to operator-mlab-sandbox/prometheus/blackbox-targets/ssh806_ipv6.json.</Details></Error>

When prometheus tries to parse this file as if it were a file-source it fails.

gcp-service-discovery should not write the content of missing files.