[w3c/IndexedDB] Return explicit failure from object store retrieval operations (Issue #432)

As discussed in #430 - 

In https://w3c.github.io/IndexedDB/#object-store-retrieval-operation the operation is noted to _"return undefined, an ECMAScript value, or an error (a DOMException)"_

The problem with this is that DOMException (and JS errors) are [Serializable](https://webidl.spec.whatwg.org/#idl-DOMException) so to [the caller](https://w3c.github.io/IndexedDB/#async-execute-request) it is ambiguous if a stored exception is successfully returned, or if the operation failed and is returning an error.

One simple approach would be to return "_success and a value (an ECMAScript value or undefined), or failure and an error (a DOMException)_", with specific steps saying e.g. "return success and value" or "return failure and a FooException"  and then have the caller operate on "if _result_ is failure" rather than "if _result_ is an error". (We could make this a [tuple](https://infra.spec.whatwg.org/#tuples) but I think that's overkill.)

All [database operations](https://w3c.github.io/IndexedDB/#database-operations) should be updated to be consistent. For example, in [object store storage](https://w3c.github.io/IndexedDB/#object-store-storage-operation) the phrasing of _"... then this operation failed ..."_ should be updated to explicitly return failure.

@abhishek-shanthkumar or @SteveBeckerMSFT - any interest in putting up a PR for this?

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