Improve error messaging around endpoint 404s

By default, if there is no `--endpoint` or service url provided, the language server will try to introspect against `http://localhost:4000`. If this introspection fails, the user sees a "request to http://localhost:4000 failed" error, but without any explanation as to _why_ it was looking at localhost to begin with.

We should recognize when a url isn't passed in, and provide a helpful error when introspection against the default url fails.

Start by tracing the language server's introspection path when no --endpoint or service URL is provided, including where the default http://localhost:4000 URL is selected and its failure is reported. Done means a failed request to that default URL explains why localhost was used, while an explicitly supplied URL retains appropriate error messaging.

Written by the indexing model from the issue text.