syn2mas: panic in writer tasks can lead to stalling

If a syn2mas MAS writer task panics, it doesn't shut down properly, but depending on the exact failure:

- if all tasks panic, I suspect it will generally get stuck waiting for them to be ready to accept new tasks
- if not all of them panic, it will be detected that some panicked — at the end

_Originally posted by @reivilibre in https://github.com/element-hq/matrix-authentication-service/pull/3878#discussion_r1930870108_