Improve handling of problematic input footprints

[Ylannl]

The SimplifyPolygons function in Vector2DOpsGEOS.cpp has a few checks for problematic footprints, due to invalidity or a multigeometry after the simplify function (See #99). While this is currently detected, the only reasonable way to handle this is to skip the entire parent tile for further processing.

• We should improve the handling of such features so that only the problematic features are skipped. Also there should be more helpful error messages that tell the user exactly which features are problematic (currently only a translated WKT is outputted).
• in addition perhaps some cases like Crash on footprint with spike #99 can be properly handled if we implement support for multi-polygons in the reconstruction code.