Add error highlighting.

[aoripov]

Add source error highlighting of all errors that have source references. User should hover over the error in error viewer and part of source that contains error should be highlighted.

[eluzhnica]

@aoripov What is the state on this one? Is this the one that is blocked by the parser?

[aoripov]

The state is frozen. We stopped, because we thought that parser should be developed further and only then we can continue.