Semi-fix bug #10013 (CSS Quick Edit provider does not respond if project contains any non-UTF8 CSS/LESS/SCSS files)

Semi-fix for bug #10013 -- Silently ignore files that can't be read, and continue the search anyway (warning logged to console, but no user-visible error shown). Similar to what we've always done for Find in Files.

With unit test, and matching test for the pre-existing same behavior in Find in Files. Also clean up dated use of `waitsFor()` in the CSSUtils test spec.
Usually the argument form here is better since you can inspect the object more deeply in dev tools after the fact.  The one exception I've noticed is Error objects, where the dev tools rendition inexplicably doesn't show the error message, even though Error.toString() does.  I'll change the one in CSSUtils to match this.