auto-t: hostapd: raise exception upon invalid config

If the config file passed in is not found we would continue and
eventually something else would fail. Instead immediately raise an
exception to be more clear on what is actually failing.

autotests/util/hostapd.py

@@ -37,6 +37,9 @@ class HostapdCLI:
                     interface = intf
                     break
 
+        if not interface:
+            raise Exception('config %s not found' % config)
+
         self.ifname = interface.name
         self.socket_path = os.path.dirname(interface.ctrl_interface)