Improve error/warning error format

[nicolasstucki]

Use red for error, yellow for warnings and blue for info.

Previously all warnings looked like errors:

Now we can easely diferentiate them (blue is still kept for info):

[anatoliykmetyuk]

I think blue is a bit hard to read on the black background. Btw, same goes to the current Dotty error reporting. I always select the Dotty error output before reading it. What do you think about experimenting with a whiter color?

[nicolasstucki]

I agree that the color pallet is not ideal. We should have a look into it later. In that case will need to find globally all uses of color and change them to be consistent. It is also important to check that the colors also look good on white.