htmlreport: fix #5963, handle inconclusive messages and underlay them in grey color.

It'll now look like this:
![cppcheck](https://cloud.githubusercontent.com/assets/476013/3619646/071506a2-0df7-11e4-936b-ed180e6a9239.png)


I hope the solution isn't too hacky, I'm still a python beginner, doing python stuff since this semester :)
Note that the indentation is incorrect here.