support throwing from response

[gsimko]

It would be nice to support throwing errors from tracker.response. It's almost working today but the setTimeout() that surrounds the internals of the Promise in Tracker._handle puts the exception in the wrong context and hence it's not raised through the same stack as a regular db error is. I'm uncertain what's the purpose of that timeout is - would it be possible to remove it?

Use case: I have an array of objects that I want to respond with during mocking. Some of these are pg.DatabaseError objects, which would require some complicated orchestration with simulateError to emit at the right time - if at all doable.
For example I have test queries that fail on first attempt due to race conditions (using serializable transactions) and succeed on the second one. It's hard to describe such a scenario with the current API as the query/binding/etc is the same, the only difference is in which order they are called.