Emit error tokens when there's a group mismatch.

Also don't panic/recover, as we no longer use panic to report "real"
errors.

Fixes #295.

formatters/html/html.go

@@ -91,11 +91,6 @@ func (h highlightRanges) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }
 func (h highlightRanges) Less(i, j int) bool { return h[i][0] < h[j][0] }
 
 func (f *Formatter) Format(w io.Writer, style *chroma.Style, iterator chroma.Iterator) (err error) {
-	defer func() {
-		if perr := recover(); perr != nil {
-			err = perr.(error)
-		}
-	}()
 	return f.writeHTML(w, style, iterator.Tokens())
 }