(bug 26486) ResourceLoader modules with paths to nonexistent files cause PHP warnings...

(bug 26486) ResourceLoader modules with paths to nonexistent files cause PHP warnings/notices to be thrown. Worked around all of them using file_exists() checks, and introduced a safe wrapper around filemtime() that checks for file_exists(). The only warning I couldn't squash is "Warning: array_map(): An error occurred while invoking the map callback" but that's due to a bug in PHP, which I've reported at https://bugs.php.net/bug.php?id=55416