Inform user about which subexpressions were unsamblable

[pavpanchekha]

In most cases we can localize unsamblability to a specific operator (the one that got a too-high precision assigned, I think?). We should expose that somehow in the API, so that we can give that to the user in warnings or similar.

[AYadrov]

Yeah, maybe at explain? Otherwise it would slow down runtime probably as somebody might use REPL's eval for performance