Port Osmose "Unconnected waterway or wrong way flow" check to JOSM validator

[liotier]

In large complicated hydrographic networks, waterway flow direction consistency is a common issue, as the number of errors reported by Osmose attests: ​http://osmose.openstreetmap.fr/en/errors/?item=1220 - reducing the number of such errors that reach the Openstreetmap database would be valuable.

The Osmose analyzer that reports those errors is published at ​https://github.com/osm-fr/osmose-backend/blob/master/analysers/analyser_osmosis_waterway.py

I believe that this error should be a warning because there are rare but legitimate cases for a waterway an hydrographic network not being consistent with a fully hierarchic model taking root into the ocean.