API errors might not roll back DB transactions

[willdurand]

(this is from some investigations on last night's production bug)

It looks like DRF only re-raises an exception when its handler returns None.
Our custom_exception_handler returns a 500 for non-API exceptions instead,
so the exception never propagates out of the view. When that happens, the
transaction is committed.

I believe we should rollback instead.

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