Throw error when user not found in ExperimentController

[KD1712]

No description provided.

[RidhamShah]

These changes should be on all versions of ExperimentClientController.

[VivekFitkariwala]

@bcb37 @danoswaltCL, At present we are fetching the user doc in each HTTP request of Experiment Client and that is redundant. In this PR is moving that in the middleware and the error will be thrown at the place where the document is fetched instead of the service which is incorrect.
We can have this middleware in the latest v5 version and not touch the service for backward compatibility.

cc - @KD1712

[danoswaltCL]

Another consideration is that this will break current non-client-library usage of the API, such as Livehint AI. They will need to begin sending this in the header for all calls.

[danoswaltCL]

Let's wait on merging this in, see my comments in #1606

[danoswaltCL]

holding off on this one, going with existing 404 errors in the individual endpoints still for now

[danoswaltCL]

Needs to be only a part of the v6 controller. see: #1851 @ppratikcr7 @VivekFitkariwala

[ppratikcr7]

Closing this as its handled in a new PR with v6 client controller.