Merge: Hint to bad class names

Add hints when a class name cause an error.

Basically 3 cases are considered: spellign, bad visibility and not imported. e.g.

~~~
Error: class `Fail` not found in module `error_unk_class`.
Error: class `Boolean` not found in module `error_unk_class`. Did you mean `Bool`?
Error: class `core::list::ListNode` not visible in module `error_unk_class`.
Error: class `POSet` not found in module `error_unk_class`. Maybe import `poset`?
~~~

Pull-Request: #2034
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>