Is your feature request related to a problem? Please describe.
The Agones allocator has 2 possible errors that it can return (among others): no game server found or failed to update the game server CR. Both are depicting the same situation (basically no game server was found to make the allocation happen) but result in different outcomes from the public API which is confusing. The confusion comes from the fact that the API response will be nil and the error code will be translated into an internal error.

Describe the solution you'd like
Create ErrGameServerUpdateConflict and use it here and treat the new error similarly to ErrNoGameServer here thus ending the gsa with the state GameServerAllocationUnAllocated.

Describe alternatives you've considered
Failure to update the CR should result in the same outcome as no server found: returning ErrNoGameServer here

Additional context
None

Link to the Agones Feature Proposal (if any)
None

Discussion Link (if any)
None