Conduit: Print useful error when setting non-existing object subscribers
Summary:
Passing bogus values to the Conduit endpoints 'subscribers.remove', 'subscribers.add', 'subscribers.set', do not silently fail or do not emit confusing `Edges are not available for objects of type '????'!`, but throw a useful error message on empty/invalid values like
`"ERR-CONDUIT-CORE: <maniphest.edit> Subscriber transactions must be existing PHIDs or usernames (found key \"nonsense\" on transaction of type \"core:subscribers\")."`
Closes T16311
Test Plan:
* Edit the subscribers of an existing task in the web UI, e.g. add/remove existing users and projects
* Run a bunch of API calls passing valid and invalid usernames/project tags and see that they now fail with informative error output telling you which value is invalid, e.g.
```
echo '{"transactions": [{"type": "subscribers.add", "value":["user1","user2","someExistingProjectName"]}],"objectIdentifier":"T3955"}' | /var/www/html/phorge/arcanist/bin/arc call-conduit --conduit-uri http://phorge.localhost --conduit-token "cli-xxx" maniphest.edit --
echo '{"transactions": [{"type": "subscribers.add", "value":["PHID-xxxinvalid","user1",""]},{"type": "subscribers.remove", "value":["user4","usernonexisting"]}],"objectIdentifier":"T3955"}' | /var/www/html/phorge/arcanist/bin/arc call-conduit --conduit-uri http://phorge.localhost --conduit-token "cli-xxx" maniphest.edit --
echo '{"transactions": [{"type": "subscribers.add", "value":["user9"]},{"type": "subscribers.remove", "value":["user3","usernonexisting"]}],"objectIdentifier":"T3955"}' | /var/www/html/phorge/arcanist/bin/arc call-conduit --conduit-uri http://phorge.localhost --conduit-token "cli-xxx" maniphest.edit --
echo '{"transactions": [{"type": "subscribers.add", "value":["user9"]},{"type": "subscribers.remove", "value":["user3"]}],"objectIdentifier":"T3955"}' | /var/www/html/phorge/arcanist/bin/arc call-conduit --conduit-uri http://phorge.localhost --conduit-token "cli-xxx" maniphest.edit --
```
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16311
Differential Revision: https://we.phorge.it/D26696