Consider returning a promise from saveImpression

[apasel422]

The difference in shape of the exception handling here makes me concerned that we won't be able to retrofit a Promise return for saveImpression. Should I be concerned?

Originally posted by @martinthomson in #178 (review)

[apasel422]

For consistency with measureConversion and future-compatibility with some kind of output, this makes sense. I'm not sure which of the following is better for compatibility:

Promise<void> saveImpression();

dictionary PrivateAttributionImpressionResult {};
Promise<PrivateAttributionImpressionResult> saveImpression();