Fixed issue with non-existent reviewers file

An error occured if the reviewers file did not exist or did not contain
valid JSON data. This should not happen anymore as now, an empty set of
reviewers is assumed in that cases.

Signed-off-by: Lars Kiesow <lkiesow@uos.de>
Added notice about automatic reviewer selection

Signed-off-by: Lars Kiesow <lkiesow@uos.de>