[web] Make failed boolean in PendingMultimediaUpload

Summary:
The same as D4890, but for web. Since we were already using failed as a truthy/falsy type (with the exception of when we wanted to store an error message), converting it to a boolean makes more sense.

Doesn't actually depend on D4890, but adding it to the stack since they're closely related.

Test Plan: Searched through the codebase for all usages of PendingMultimediaUpload for this refactor. Sent some messages on web and console.log(...)'d some things, and everything looked as expected.

Reviewers: atul, tomek, jacek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D4891