Improve error message for "Sync not yet implemented"

[notrab]

When attempting to use offline writes with sqld, the following error is returned:

Syncing database ...
thread '<unnamed>' panicked at src/errors.rs:22:13:
not yet implemented: err = Sync(PullFrame(404, ""))
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
[1]    47439 abort      node index.mjs

You can somewhat decipher with the "not yet implemented" error that this isn't using a version of turso-server, because we know it's implemented but it wasn't immediately clear. We can do provide a better message here.