conf: don't repeat error message when adding sourcedir source

conf: don't repeat error message when adding sourcedir source

When a source from a configured sourcedir cannot be added (e.g. it is a
duplicate of another source), log the error message only on the first
attempt adding the source, until the source is removed and added to a
sourcedir again.

This avoids spamming of the system log with error messages if the
reload sources command is called frequently (e.g. from a DHCP renewal
networking script).