Placeholder for validation error messages

[runceel]

This PR is for issue #136 .

Current behavior

class Foo
{
  [Required(ErrorMessage = "{0} is required.")]
  [Display(Name = "SomeProperty")]
  public ReactiveProperty<string> Name { get; }
  public Foo()
  {
    Name = new ReactiveProperty<string>()
      .SetValidateAttribute(() => Name);
  }
}

var f = new Foo();
var errorMessage = f.GetErrors("Name").Cast<string>().FirstOrdefault(); // Value is required.

Expected behavior

The error message should be "SomeProperty is required."

And if DisplayAttribute is not there, then "Name is required."