Conduit: Improve error handling calling flag.edit without objectPHID
Summary:
Avoid an ugly AphrontQueryException. Throw a proper error code instead.
```
EXCEPTION: (AphrontQueryException) #1048: Column 'objectPHID' cannot be null at [<phorge>/src/infrastructure/storage/connection/mysql/AphrontBaseMySQLDatabaseConnection.php:396]
```
Closes T16442
Test Plan: Go to http://phorge.localhost/conduit/method/flag.edit/ and press the "Call Method" button
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16442
Differential Revision: https://we.phorge.it/D26644