Revamp exception handling. ImportErrorException renamed to DeadlyImportError to avoid silent regressions. Exceptions now thrown by value, and caught by reference. Put guards in all C++ and C API functions to avoid unwanted exception propagation across module and language boundaries.

PLEASE TEST CAREFULLY IF THIS SHOULD CAUSE REGRESSIONS.

git-svn-id: https://assimp.svn.sourceforge.net/svnroot/assimp/trunk@617 67173fc5-114c-0410-ac8e-9d2fd5bffc1f