Add infrastructure that will allow us to gain visibility into errors
happening in the host frame, in specifically wrapped blocks of code, by
capturing the errors and forwarding them to the sidebar.

The initial implementation requires that the browser is able to clone
errors in `window.postMessage` calls, and currently only Chrome supports
that. In other browsers forwarding errors will fail with a warning.

 - Add `shared/frame-error-capture.js` module with functions for
   capturing errors, registering a target frame to receive them and
   forwarding errors to the frame.

 - Register sidebar application as handler for errors in host frame in
   `Sidebar` constructor and de-register it in the destructor.

 - Call `handleErrorsInFrames` in `sidebar/util/sentry.js` to handle
   errors from host frame by sending them to Sentry.