Error message are bad on type error

[irevoire]

While playing a bit with the type system I realized that our error messages are pretty bad.
We don't even point to any specific location. Here's an example:

fn truc<A: Dim>(a: A) = 12
truc("hello")

Returns:

error: Could not solve the following constraints:
  String: DType
.. while trying to infer types in the (elaborated) statement:
  truc("hello")

 = Consider adding type annotations to get more precise error messages.

Interpreter stopped

From what I see we're able to retrieve the truc("hello") so we should at least be able to point to that location.