fix panic on duplicate remoteuser key

[ghost]

this changes handleFetchCollectionInbox to log all errors after
attempting to insert an actor in the remoteusers table. previously
checking for all errors except duplicate keys would cause a panic if
an actor made a request to follow while already having followed.

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closes #159

[ghost]

to test the fix use a local isntance and try something like the following:

curl -H "Content-Type: application/json" \
    --request POST \
    --data '{
    "@context": "https://www.w3.org/ns/activitystreams",
    "id": "https://my-example.com/my-first-follow",
    "type": "Follow",
    "actor": "https://mastodon.technology/@robjloranger",
    "object": "https://mastodon.social/users/Mastodon"
    }' \
    http://localhost:8080/api/collections/rob/inbox

The collection in the target inbox needs to exist and the actor may be any activity pub actor that is live.

[thebaer]

Wasn't able to replicate the crash with this (or a few other things I tried), but this should at least fix it. Thanks! Merging now.