Merge: typing: include a hook to enable more precise error information on 'expected...

Merge: typing: include a hook to enable more precise error information on 'expected expression' errors

The point is to help to solve the second part of #1392

Before:

~~~
print (-1).to_s
^
test.nit:2,1--10: Error: expected an expression.
~~~

After:

~~~
print (-1).to_s
^
test.nit:2,1--10: Error: expected an expression to be the receiver of `to_s`
~~~

Pull-Request: #1963
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>