Handle fail of a non-default server in an app test

It is possible that a crash detector of a non-default instance calls for
`kill_current_test()` and the greenlet that executes the current test
will be killed. So it'll not set the returncode.

See #252 for reproducer.

Part of #65
Part of #157

Author: Totktonada

lib/app_server.py

@@ -78,7 +78,7 @@ def execute(self, server):
             # See a comment in LuaTest.execute() for motivation of
             # SIGKILL usage.
             ts.stop_nondefault(signal=signal.SIGKILL)
-        if retval['returncode'] != 0:
+        if retval.get('returncode', None) != 0:
             raise TestExecutionError