Better handling of quota errors when saving attachments to server

[dstillman]

If I'm at my storage quota, I get a misleading error message that suggests the whole save has failed:

And I get two broken attachments, since the attachment items are created but the file requests fail with 413s, and the attachment items aren't deleted after.

The proper behavior here would be red X for the attachments and automatic DELETE on the failed attachment items, plus ideally a message below about the quota.

Originally posted in #535 (comment)