Inform clients when Preview support enabled for an incompatible version.

[rgrunber]

• Fixes Notify clients that enabling preview features on incompatible compliance level fails the build #3131

[jdneo]

I'm thinking about how should the client side deal with this notification? It makes things clearer than only log it in the log file. However, users might still have no idea what should they do after receiving this notification?

[mickaelistria]

IMO, the expectation of end-users is that preview flags just get silently ignored when they're not targeting latest version and that they shouldn't bother with them. It's the pattern we've seen in mulitple place in JDT and the Javac incubator.

[rgrunber]

At least for vscode-java, I have a client-side PR (local) that catches the event notification (like we do for incompatible Gradle + JDK versions) and shows the error message : "Preview features enabled at an invalid source release level ..., preview can be enabled only at source level ...", and a link to the build configuration file to change it.

Particularly on #3131 (comment) , if the clients do nothing with it, at least more is logged, at least until we properly address redhat-developer/vscode-java#1111 (disappearing errors when text document opened, if coming from workspace diagnostic handler).

As for Mickael's suggestion of ignoring, I kind of like it except for the fact that we're opening ourselves up for users to later complain that "preview features don't work". Eclipse doesn't actually silently ignored the error though. If you force preview features, it will complain : #3131 (comment)