* installation URL is preferred - same behavior as before
* @updateURL is used when the style was drag'n'dropped into the manage page
because there's no real URL in this case
* install-usercss page shows the new update URL, which is set as per the above,
under the checkbox that enables updates
Promise mode [default]:
- rejects on receiving {__ERROR__: message} created by
background.js::onRuntimeMessage
- suppresses chrome.runtime.lastError
by browserAction.setText which lacks a callback param in chrome API
Callback mode:
- enabled by passing a second param
- doesn't suppress chrome.runtime.lastError
