BUG/MINOR: config: Warn only if warnif_cond_conflicts report a conflict

BUG/MINOR: config: Warn only if warnif_cond_conflicts report a conflict

When warnif_cond_conflicts() is called, we must take care to emit a warning
only when a conflict is reported. We cannot rely on the err_code variable
because some warnings may have been already reported. We now rely on the
errmsg variable. If it contains something, a warning is emitted. It is good
enough becasue warnif_cond_conflicts() only reports warnings.

This patch should fix the issue #3305. It is a 3.4-dev specific issue. No
backport needed.