[Interp] Improve error message / diagnostics infrastructure

[ngernest]

As Nikil mentioned in #177:

as the complexity and number of error messages we have increase, its clear that the current error handling and diagnostic handler architecture, at least in the interpreter, is a little unwieldy. It is probably low priority, but we could put in issues for a rewrite of these two that couples them together better to eliminate code duplication and make the error handling a lot less painful to implement / change.

This is relatively low priority though, hence the Chore label.