If file could not be accessed on disk, ignore trim_modified_lines_only setting

Occasionally, TrailingSpaces throws an exceptions in the `codecs.open` function. The most common case is when you are editing a file that was opened from within a `.sublime-package` file, like, for instance, your default key bindings file. It doesn't really have a major impact, but I like to keep my console free of exceptions from plugins. This change should fix it. The only caveat is that, for such views, the trim_modified_lines_only setting will be ignored, simply because there doesn't seem to be a good way to take a snapshot of the view.

Also added a minor optimization wherein a snapshot of the file is not taken unless trim_modified_lines_only is set.