R1:29b4fad94173

Get rid of "throwResult()" for control flow in MFA factors

Summary: Depends on D20034. Ref T13222. This is just cleanup -- I thought we'd have like two of these, but we ended up having a whole lot in Duo and a decent number in SMS. Just let factors return a result explicitly if they can make a decision early. I think using `instanceof` for control flow is a lesser evil than using `catch`, on the balance.

Test Plan: `grep`, went through enroll/gate flows on SMS and Duo.

Reviewers: amckinley

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