Refactor error reporting in `TypeInference`

Refactoring of error reporting is motivated by the following needs.

- To solve the issue #19 we need to run the type-checker of find the instance of method `toString`. However, this could raise some type-errors, that should not be exposed to the user, or at least delayed until a proper message is displayed (about being unable to use `toString` method).
- As suggested by @kwasielewski it would be nice to have good infrastructure for attaching notes to error messages.
- We should set up the scene for managing warnings: being able to disable them or raise them to errors.