(1) A bug occurred in which the Hide n' Seek UI failed to display on
Indeed and LinkedIn, and then continued to fail to display even after
reloading the tabs. Two uncaught promise rejection errors were logged:

* "No tab with id: ##########."
* "Frame with ID 0 was removed."

Although I am not certain that these errors were the cause of the
missing Hide n' Seek UI, I decided to simply handle rejections by
adding a safeAwait utility function that handles the promises returned
by several Chrome APIs, such as scripting.executeScript,
scripting.insertCSS, etc.

(1) async was removed from the addListener callback because it was
unnecessary.

(2) Removed unnecessary storage arguments from calls to popup start
methods

(1) async was removed from the runtime.onMessage.addListener callback
because it is unnecessary.