DO NOT MERGE ANYWHERE: Add file write error checks to config_save

It's better to leave old config file instead of
overwriting with broken one.

Fixes problem when disk is full and we can't write
more data to it.

(Change amended to use "old" style LOG_ERROR macro)

Issue: KIONE-3099
Change-Id: Ifdb641ba7f3140655f9ec371e53b11a17484f27a