Refactor error reporting in TypeInference

[ppolesiuk]

Refactoring of error reporting is motivated by the following needs.

• To solve the issue REPL pretty-printing based on toString method #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.