Improve status indicators and error handling

[jubeira]

The status indicators work fine when everything goes well, but they are not so great for debugging.
To note:

• ROS indicator goes from paused to OK; it could be set to loading when the connection is checked
• The case where no master is found can be handled better
• onTangoStatus only checks for "OK" status; it's not so hard to fix the indicator to reflect other cases. The problem is that this method is usually called more than once, and it may fail at first.