err.indexOf is not a function

Created by: RavanH

https://github.com/duniter/duniter/issues/616

Created by: coveralls

Coverage decreased (-0.2%) to 84.445% when pulling 4a1c233f on RavanH:dev into d8f7da66 on duniter:dev.

Created by: M5oul

Thanks, but it will not solved the issue as explained on related ticket.