Improve the error shown when an RPC return value fails serialisation

[mbirtwell]

ARTIQ Pull Request

Description of Changes

I've encountered failures with RPC return values failing to serialise from time to time. e.g. returning a different type than annotated from the RPC function. Making sure that the name of the RPC function called is included in the error makes tracking down the problem much easier.

Related Issue

Type of Changes

	Type
✓	:hammer: Refactoring

Steps (Choose relevant, delete irrelevant before submitting)

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