Better ErgoTree evaluation error messages

[greenhat]

Motivation

The error message only describes what IR node evaluation failed without any context information.

The following error message communicates the context better:

...
val v5 = OUTPUTS(2)
                ^^^ - index out of bounds
...

For a deserialized ErgoTree the source code can be generated by pretty printer #371 .

Solution

Ideally, we'd want a pretty printed tree with evaluation results(values) printed at ValDef's.

One of the options is to add a source span to every node and pretty print the tree on an error at the top-level evaluator (ErgoTree).