fix(data): make DataServiceError extend from Error

[rob4226]

I was kind of surprised when DataServiceError instanceof Error returned false but then I saw the note in the source code (4 years ago) about how dse instanceof DataServiceError didn't work in unit tests when extending from Error. As long as super() is called in the child class constructor when extending from Error, then instanceof should work fine. I made the change, then ran the unit tests and they all passed so it seems it's ok.

This is not a big deal but it's usually good form to have Error in the inheritance chain of custom errors. I don't believe this is a breaking change.

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[timdeschryver]

Weird, I wonder why this failed before.
It seems to work now 👍