SAMPHubServer should raise an exception if another hub is already running

Update: This issue was moved to pyvo following https://github.com/astropy/pyvo/pull/729

---

At the moment the current behavior could be considered a bug:

```python
In [8]: from astropy.samp import SAMPHubServer

In [9]: hub = SAMPHubServer()

In [10]: hub.start()
INFO: Hub set to run with Web Profile support enabled. [astropy.samp.hub]
/Users/tom/Dropbox/Code/Astropy/astropy/astropy/samp/lockfile_helpers.py:114: SAMPWarning: Another SAMP Hub is already running. Start-up process aborted.
"aborted.", SAMPWarning)
INFO: Hub started [astropy.samp.hub]

In [11]: hub.is_running
Out[11]: True
```

Start at the SAMPHubServer entry point and its start() behavior, using the example in the issue to reproduce the case where another hub is already running. Done means startup raises an exception instead of only emitting SAMPWarning and leaving is_running true.

Written by the indexing model from the issue text.