Highlight added and removed errors better

[amchiclet]

Currently, the colors are not very pleasant to look at. Consider changing the foreground/background colors. Some concrete suggestions are:

1. Choose a better color for removed errors (e.g., a better green or even blue).
2. Experiment with different background colors to highlight the removed and added errors.
3. If we decide to change the background colors, use border lines instead of grey/while to separate the errors.
4. Experiment with adding a new icon, because when an error is selected, the highlighting is not visible.

[amchiclet]

Used light bulb icons to indicate errors in the error-centric view.