fixed PolygonRegionLoader GWT compatibility and error report

removed call to Class#getSimpleName() for GWT compatibility
throws GdxRuntimeException instead of logging to Gdx.app.error(..)
Use UTF-8 for Hiero settings

Closes #2308
Merge pull request #2328 from dermetfan/master

fixed PolygonRegionLoader GWT compatibility and error report
Merge branch 'master' of https://github.com/libgdx/libgdx into iap