Exporters: raise errors if output files cannot be opened.

code/STLExporter.cpp

@@ -57,6 +57,10 @@ void ExportSceneSTL(const char* pFile,IOSystem* pIOSystem, const aiScene* pScene
 
 	// we're still here - export successfully completed. Write the file.
 	boost::scoped_ptr<IOStream> outfile (pIOSystem->Open(pFile,"wt"));
+	if(outfile == NULL) {
+		throw DeadlyExportError("could not open output .stl file: " + std::string(pFile));
+	}
+
 	outfile->Write( exporter.mOutput.str().c_str(), static_cast<size_t>(exporter.mOutput.tellp()),1);
 }