is_running() works incorrectly, if execution fails with an exception

[okalachev]

If state machine's state fails with an exception, _is_running flag is not lowered, although the state machine obviously is not running.

Example code:

from smach import StateMachine, State

class FailingState(State):
    def execute(self, ud):
        0/0  # exception

s = StateMachine(outcomes=[])
with s:
    StateMachine.add('foo', FailingState())

try:
    s.execute()
except:
    pass

print 'running', s.is_running()

Output:

<...>
ZeroDivisionError: integer division or modulo by zero
<...>
running True

Isn't is an issue?