Prevent error boundary from masking a test error

[bvaughn]

This code in ReactFiberScheduler can suppress "unexpected console warning" Errors thrown by our custom toWarnDev matcher, so I've changed the matcher to report those Errors to Jest as failed assertions instead. This means that we won't halt as quickly as before, but we'll still fail within the bounds of the expect().toWarnDev() callback so it's probably okay.

Before this change, the following test would unexpectedly pass:

class Parent extends React.Component {
  state = {};
  componentDidCatch(error) {
    this.setState({error}); // Render again
  }
  render() {
    return this.state.error ? null : <Child />;
  }
}

function Child() {
  const error = new Error('componentDidMount error');
  error.suppressReactErrorLogging = undefined;
  throw error;
}

ReactNoop.render(<Parent />);
expect(ReactNoop.flush).toWarnDev([]);

After this change, it fails unless the warning is explicitly expected or the entire block is wrapped in an expect(...).toThrow() block.

Note that the test would also have passed if we replaced expect(ReactNoop.flush).toWarnDev([]); above with ReactNoop.flush(); because of the way our default console "spy" queues Errors up to report in an afterEach block.

This PR also slightly improves the pretty printing message format in this case for when there are no remaining expected warnings or multiple ones. For example:

[gaearon]

Looks reasonable to me