Refresh completion list if RBS file has a syntax error

[soutaro]

This helps generating better result in the following scenario.

class Foo
  attr_reader name: 👈 Start completion here
end

The RBS file has a syntax error and it starts completion with nil nesting context, while the user expects ::Foo.

Setting is_incomplete: true in this case forces generating a new list after user typing some character, that would make the syntax correct.