Harden ResponseCache (and fix #9507)

[ShadowJonathan]

This fixes #9507 by making ResponseCache additionally not cache the result when;

• The original function throws an exception.
• One of the conditional callables, upon executing, throws an exception.

Previously, the latter could cause the cache not to be evicted inside the errback of the ObservableDeferred, because these conditionals would be evaluated (and have an exception thrown) before a callLater or immidiate cache eviction could be done, this what caused #9507 (most likely).

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
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Signed-off-by: Jonathan de Jong <jonathan@automatia.nl>

Fixes #9507

[ShadowJonathan]

(I also split the functions into class-level private methods, because it was getting too crowded with 3 local functions)

[richvdh]

for links: I've argued in #synapse-dev and https://github.com/matrix-org/synapse/pull/9358/files#r585481702 that we should only support one conditional per key, which simplifies this code.

[ShadowJonathan]

I'll add that to the "small changes to ResponseCache PR", this one is focused on fixing #9507, i dont wanna drag everything else into here.

[ShadowJonathan]

Created #9525 for this

[ShadowJonathan]

Folded into #9739