After migrating the the `browser.js` file to ES2015, the `Browser` class has now fields with circular references (like `emitter`),
which breaks the Karma JSON reporters available starting with Karma 2.0.1.
This commit removes the circular dependencies by adding a `toJSON()` method to the `Browser` class,
which produces a similar result to the browser object before the migration.

Fixes #3075

Rather than catching and discarding the error if no error handler is passed, let the promise package chain it. This allows use to have a .catch() follow the then().

… unused promise (#3060)

* fix(browser): ensure browser state is EXECUTING when tests start

Browser state is EXECUTING when it actually started to execute tests. This state change is triggered by client on actual tests execution start.

Introduced an additional browser state CONFIGURING

The CONFIGURING state means that the browser is not just CONNECTED for tests, but someone has requested tests execution (and provided a config file). But the provided config file is not yet processed, configuration is not applied or the tests execution is not yet started and we have not received the first event from the remote browser, so the browser object is not yet at EXECUTING state.

Refactored browser state names: renamed READY -> CONNECTED

Fixes #1640