[Interp] Improve error message / diagnostics infrastructure

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.

Start by reviewing the interpreter's current error handling and diagnostic handler architecture. Identify where their responsibilities overlap and where code is duplicated. The work is done when the two are coupled more coherently, duplication is reduced, and implementing or changing diagnostics is less cumbersome.

Written by the indexing model from the issue text.