More multi failures

Created by: myronmarston

This PR addresses a bunch of deficiencies I found in how rspec-core handles aggregate_failures. This includes a fix for #1966 (closed), although the bigger fix is for how it handles a case like this:

it "has failures", :aggregate_failures do
  expect(x).to eq(2)

  raise_if_x_is_not_2(x)
end

Previously, the error from the last method call was reported as the example's main exception and the expect(x).to eq(2) failure was reported using a "An error occurred in an around hook" message, which was confusing and generally a bad experience. The problem was that aggregate_failures handles any exceptions that happen within the block, but when using metadata it wraps rspec-core's handling of errors so the failure aggregator doesn't see the error that happens. I had to change how we handle aggregate_failures a bit to deal with this.

I also introduced a RSpec::Core::MultipleExceptionError to more generally handle multiple exceptions for one example.

Finally, pending interacted weirdly with aggregate_failures so I dealt with that, too.

Note that this builds on top of rspec/rspec-expectations#800.

I'm hoping to release 3.3 once this is merged.

/cc @rspec/rspec