Playground: Make "Ambiguous Alternatives" visible for the user

[Lotes]

Here's an example of a user writing a grammar in the Playground and not seeing the "Ambiguous Alternatives Detected" parser error, which is only printed in the console:
eclipse-langium/langium#1259

We should make such parser validation errors visible in the Playground UI.

Originally posted by @spoenemann in #130 (comment)