SnapshotUploader::upload_from_file should not retry file permission errors

[iliana]

I am currently working on an image builder (go figure!), which happens to write images with root:root -rw------- permissions. Running coldsnap as my regular user doesn't work because it can't read it, but it doesn't tell me that¹; it just hangs at 0% progress.

This is related to #216 and similar, but there are certain classes of errors that should just not be retried.

Unfortunately coming up with a list of errors that should never be retried is pretty obnoxious so I might suggest attempting to open the file for reading before even calling ebs:StartSnapshot; while ENOENT will occur at the fs::metadata call, that call doesn't catch this type of permissions issue.

Footnotes

1. actually it does if i'm patient and willing to wait ~2.5 minutes apparently longer but i'm not ↩