generated: unpackaged options should display a better error message

[MattSturgeon]

Currently we rely on a no-default package option when there is no known package in nixpkgs for a none-ls, efmls, or LSP server.

This results in a fairly generic error:

error: The option `plugins.lsp.servers.<name>.package' was accessed but has no value defined. Try setting the option.

Instead we should handle these cases with our own custom assertion so that we can provide a more useful error message.

See prior discussion in #2405