BGE: Navmesh fixes and improvements

The navigation mesh functionality was broken for quite a while. This patch
contains fixes: recalculating tesselations before getting the number of
tesselation faces (it otherwise returned 0) before calculating the navmesh,
and calling `DM_ensure_tessface()` on the navmesh's `DerivedMesh` object
(which fixes visualisation in Blender). This allows one to create a new
navmesh, which also works in the BGE.

Furthermore, the patch adds several return values, and shows more error
messages when things go wrong. In several places in the navmesh creation
code, return codes weren't checked and errors silently ignored.

Reviewers: nicks, brita_, campbellbarton, lordloki, moguri, panzergame

Reviewed By: panzergame

Differential Revision: https://developer.blender.org/D1435

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