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.
This change isn't needed for the fix, but in case any _other_ issues prevent CSS Quick Edit from opening/responding at all, it will give us better error logs to investigate.