Implement Conduit login prompt behavior as a pure FutureProxy, not a Future…
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Description

Implement Conduit login prompt behavior as a pure FutureProxy, not a Future-like object

Summary:
See PHI1802. Currently, we can't raise a "you must login" error in a generic way at the beginning of a workflow because we don't know if a workflow needs credentials or not.

For example, "arc help" does not need credentials but "arc diff" does.

Additionally, some actual Conduit calls do not need credentials ("conduit.ping", "conduit.getcapabilities") and others do.

Although I'd like to simplify this eventually and move away from anonymous/unauthenticated "arc", this isn't trivial today. It's also possible for third-party code to add authenticated calls to "arc help", etc., so even if we could execute these tests upfront it's not obvious we'd want to.

So, for now, we raise "you must login" at runtime, when we receive an authentication error from Conduit.

This got implemented for Toolsets in a well-intentioned but not-so-great way somewhere in wilds/experimental, with an "ArcanistConduitCall" that behaves a bit like a future but is not really a future. This implementation made more sense when ConduitEngine was serving as a future engine, and FutureProxy could not rewrite exceptions.

After the Toolsets code was first written, ConduitEngine has stopped serving as a future engine (this is now in "HardpointEngine"). Since HardpointEngine needs a real future, this "show the user a login message" code gets bypassed. This results in user-visible raw authentication exceptions on some workflows:

[2020-06-30 21:39:53] EXCEPTION: (ConduitClientException) ERR-INVALID-SESSION: Session key is not present. at [<arcanist>/src/conduit/ConduitFuture.php:76]

To fix this:

Allow FutureProxy to rewrite exceptions (see D21383).
Implement "ArcanistConduitCall" as a FutureProxy, not a future-like object.
Collapse the mixed-mode future/not-quite-a-future APIs into a single "real future" API.

Test Plan: