Add better styling for warning & error messages

[skerit]

Right now console.error messages get a very prominent Error prefix, but that just indents the line too much.

Warning messages (console.warn) only get a yellow exclamation mark in the gutter. They should actually be less pronounced. Maybe make the entire line a slight gray colour instead of pure white?

The gutter icons might also need work: they're very, very subtle. Maybe remove or invert them?