Track placeholder user reassignment system failures

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With !182679 (merged) we implemented event tracking for several placeholder user reassignment events.

At the moment we track failure to propose_placeholder_user_reassignment and accept_placeholde_user_reassignnment

We now want to add tracking for system failures on the following events:

• failure to keep_as_placeholder
• failure to keep_all_as_placeholder
• failure to cancel_placeholder_user_reassignment
• failure to reject_placeholder_user_reassignment

We should be able to use the already defined fail_placeholder_user_reassignment event for cancel and reject, but will likely need to define a new events: fail_keep_as_placeholder_user and fail_keep_all_as_placeholder_user