Compiler: show warnings even if there are errors

[asterite]

Fixes #9460

Now warnings are shown even if there are errors.

I also cleaned up the code so that warnings are reported in the semantic phase, not the in the codegen phase.

Now this snippet:

abstract class Foo
  abstract def write : Int64
end
 
class Bar < Foo
  def write : Int64 # Error: method must return Int64 but it is returning Nil
    Baz.new.write
  end
end
 
class Baz < Foo
  def write # The error should be reported here!
    nil
  end
end
 
Bar.new.write

Gives this error:

In foo.cr:12:7

 12 | def write # The error should be reported here!
          ^----
Warning: this method overrides Foo#write() which has an explicit return type of Int64.

Please add an explicit return type (Int64 or a subtype of it) to this method as well.

The above warning will become an error in a future Crystal version.

A total of 1 warnings were found.
Showing last frame. Use --error-trace for full trace.

In foo.cr:6:15

 6 | def write : Int64 # Error: method must return Int64 but it is returning Nil
                 ^
Error: method must return Int64 but it is returning Nil

You can see that the warning tells you that the return type is wrong, which then connects to the error below.

If you want to fix this is another way, let me tell you that it's impossible. The idea is that abstract method errors must kick before the semantic phase, but right now they are warnings so that's impossible to do. The real solution is to make those warning be errors, but first #8766 and #8232 (which is not a bug to me, the user should define the exact method that's wanted, and later they can do a dispatch if they want) must be fixed.