notmuch restore: Print names of tags that cannot be applied

notmuch restore: Print names of tags that cannot be applied

This helps the user gauge the severity of the error.

For example, when restoring my sup tags I see a bunch of tags missing
for message IDs of the form "sup-faked-...". That's not surprising
since I know that sup generates these with the md5sum of the message
header while notmuch uses the sha-1 of the entire message. But how
much will this hurt?

Well, now that I can see that most of the missing tags are just
"attachment", then I'm not concerned, (I'll be automatically creating
that tag in the future based on the message contents). But if a
missing tag is "inbox" then that's more concerning because that's data
that I can't easily regenerate outside of sup.