Add rudimentary file error tests for freesasa_export_tree()
Add rudimentary file error tests for freesasa_export_tree()

JoaoRodrigues/freesasa,mittinatten/freesasa,mittinatten/freesasa,JoaoRodrigues/freesasa,mittinatten/freesasa,mittinatten/freesasa,JoaoRodrigues/freesasa,JoaoRodrigues/freesasa