Improved reporting for MISSING_CSS_BLOCK_OPENER

[flukeout]

See here...

In this rule we get access to the selector which is the following string...

div font-weight: bold

This is used to add the highlight above, which seems like overkill. We should modify this somewhat to maybe parse out the selector used? An easy solution would be to just look for the first space in this string and then assume that the selector is whatever precedes the space?

cc @Pomax