Cancellable: always catch internal cancellations

[majocha]

I've been running Visual Studio in debug mode and I noticed a lot of assertions related to cancellation. Cancellable.CheckAndThrow() surfaces as an internal error, despite being in the context of cancellable workflow, for example here:

fsharp/src/Compiler/Service/FSharpCheckerResults.fs

Lines 3284 to 3285 in 172dc31

	with e ->
	errorR e

This seems benign in real use, but I think is not correct. It's also annoying when trying to test things in debug mode.

The fix is to handle such things in the cancellable builder.

Update: This worked before, and I broke it. This PR just reverts relevant parts of the code.

cc: @auduchinok

[github-actions]

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[majocha]

I think this worked before and I broke it the previous time I touched this code.

[auduchinok]

@majocha Thank you! 🙂