Add a mode to "ExecFuture" that makes "resolvex()" semantics the default

Summary:
Ref T13490. "ExecFuture" can raise a "CommandException" when the subprocess fails. In most cases, this is desirable, since we usually do not expect subprocesses to fail.

Currently, "execx()" (which raises these exceptions) is the synchronous default (with the more verbose "exec_manual()" as an alternative if you expect the command may return an error code), but the Future variation has no way to hint that external resolution should raise an exception if the process exits with an error.

Since the "HardpointEngine" yield construct can resolve external futures, add a mode for this to simplify implementing "HardpointQuery" classes a bit. Without this, they either need to retain "$futures" and manually call "resolvex()", or check the "$err" result and throw some other exception, both of which are low-value boilerplate (queries that want to do this still can, of course).

This is basically like providing a hint that the futures should be resolved with "resolvex()" instead of "resolve()".

Also, make this the default behavior of a new "$api->newFuture()" wrapper.

Test Plan: Intentionally broke a command in a HardpointQuery, got a sensible exception automatically during external future resolution.

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21089