twa_graph: more test coverage

The goal is to improve coverage stats, but I discovered two issues
while doing so.

* tests/python/twagraph.py: New test case.
* tests/Makefile.am: Add it.
* spot/twa/twagraph.hh: Add fix typos in error messages.
* python/spot/impl.i: Fix broken wrappers for state_from_number and
state_acc_sets.

tests/python/twagraph.py

@@ -0,0 +1,90 @@
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2017 Laboratoire de Recherche et Développement de l'Epita
+# (LRDE).
+#
+# This file is part of Spot, a model checking library.
+#
+# Spot is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# Spot is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+# License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+
+# This file tests various error conditions on the twa API
+
+import spot
+from buddy import bddtrue
+
+aut = spot.make_twa_graph(spot.make_bdd_dict())
+
+try:
+    print(aut.to_str())
+    exit(2)
+except RuntimeError as e:
+    assert "no state" in str(e)
+
+try:
+    aut.set_init_state(2)
+except ValueError as e:
+    assert "nonexisting" in str(e)
+
+try:
+    aut.set_univ_init_state([2, 1])
+except ValueError as e:
+    assert "nonexisting" in str(e)
+
+aut.new_states(3)
+aut.set_init_state(2)
+assert aut.get_init_state_number() == 2
+e = aut.set_univ_init_state([2, 1])
+assert [2, 1] == list(aut.univ_dests(aut.get_init_state_number()))
+
+try:
+    aut.get_init_state()
+except RuntimeError as e:
+    s = str(e)
+    assert "abstract interface" in s and "alternating automata" in s
+
+all = aut.set_buchi()
+aut.new_acc_edge(0, 1, bddtrue, True)
+
+try:
+    s = aut.state_acc_sets(0)
+except RuntimeError as e:
+    assert "state-based acceptance" in str(e)
+
+try:
+    s = aut.state_is_accepting(0)
+except RuntimeError as e:
+    assert "state-based acceptance" in str(e)
+
+aut.prop_state_acc(True)
+
+assert aut.state_acc_sets(0) == all
+assert aut.state_is_accepting(0) == True
+
+aut.set_init_state(0)
+aut.purge_unreachable_states()
+i = aut.get_init_state()
+assert aut.state_is_accepting(i) == True
+
+it = aut.succ_iter(i)
+it.first()
+assert aut.edge_number(it) == 1
+assert aut.state_number(it.dst()) == 1
+assert aut.edge_storage(it).src == 0
+assert aut.edge_storage(1).src == 0
+assert aut.edge_data(it).cond == bddtrue
+assert aut.edge_data(1).cond == bddtrue
+aut.release_iter(it)
+
+aut.purge_dead_states()
+assert aut.state_is_accepting(i) == False