journal-remote: make writer_new() return negative errno on failure

journal-remote: make writer_new() return negative errno on failure

After f12b399dd6362a03379cb769954ebfb9972236ed, writer_new() may fail
with non-OOM error. Let's return the error cause, and logs the failure
in the caller side.

This also drops logs in journal_remote_get_writer(), adds its caller
typically logs the failure.