Hold error/warning messages while trying to unify call arguments

[RealyUniqueName]

Fixes #5522
Fixes #6937

class Test {
    static function f(?i:Int, fn:Array<Dynamic>-> Void){
    }

    static function main() {
        // this does not compile, with `You can't iterate on a Dynamic value, please specify Iterator or Iterable`
        f(function(res){
            for (a in res){}
        });
    }
}

This happens because it attempts to type function(res)... against ?i:Int first. Then it fails and tries to type it against fn:Array<Dynamic>-> Void and succeeds. But error messages from the first attempt already have been added to compilation result.
This PR adds a function to hold error/warning messages until it's known those messages should be submitted or discarded.

[Simn]

This is a bit too much of a central change for me to consider for 4.0. I have some concerns about the with err and the new exception type. The other issue is that we might cause inference damage that doesn't raise errors immediately.

[RealyUniqueName]

This change doesn't prevent exceptions to be raised. It only stops messages to be immediately written to Server.context.messages.

[Simn]

Brunch needs opdating.