i2p: log errors properly according to their severity

Summary:
Co-authored-by: Vasil Dimov <vd@FreeBSD.org>

This is a backport of core#29833

Bypasses a one line change from core#25355 that replaces a LogPrintfCategory by a Log. Removing that call to LogPrintfCategory allows to remove the macro after core#28318.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18268