Log user friendly error if client does not support _java.reloadBundles.command

[mfussenegger]

The result of executeClientCommand could be a map in the LSP
ResponseError format.
Previously this resulted in a ClassCastException which got logged:

class com.google.gson.internal.LinkedTreeMap cannot be cast to class java.util.ArrayList (com.google.gson.internal.LinkedTreeMap

This gave users the impression that there's a bug.
This changes the logic to instead log the message returned by the
client.

In neovim for example it results in:

Command _java.reloadBundles.command not supported on client

[rgrunber]

I actually ran into this when testing another client based on lsp4e so I know this is an issue. Normally we implement some kind of custom client-side setting under the extendedClientCapabilities property we can use to check if the client supports the command. In this case, we don't do that which seems wrong. @jdneo , this should be added in right ?

See https://github.com/eclipse/eclipse.jdt.ls/blob/73fb18c9f500d4a07548030ffc76fe89ed2c217e/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/preferences/ClientPreferences.java#L259 for an example.

So maybe something like :

if (preferenceManager.getClientPreferences().isReloadBundlesSupported()) {
    List<String> bundlesToRefresh = (ArrayList<String>) JavaLanguageServerPlugin.getInstance().getClientConnection().executeClientCommand("_java.reloadBundles.command");
}

[mfussenegger]

I think even if gated behind a capability check it would still make sense to check the result type. Clients could respond with a failure even if they support it, due to some runtime error and then you'd have a meaningful error in the log instead of the ClassCastException

[rgrunber]

Right. We do the same thing with https://github.com/eclipse/eclipse.jdt.ls/blob/73fb18c9f500d4a07548030ffc76fe89ed2c217e/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/handlers/OrganizeImportsHandler.java#L226 so using Object is probably the right approach if we don't have a capability for it.