dbserver: handle 2 moxa entries in registry with the same name

I would like the database server to fail with a more readable message than:

```python
Traceback (most recent call last):
File "C:\Instrument\Apps\EPICS\ISIS\inst_servers\master\DatabaseServer\database_server.py", line 452, in
moxa_data = MoxaData(
^^^^^^^^^
File "C:\Instrument\Apps\EPICS\ISIS\inst_servers\master\DatabaseServer\moxa_data.py", line 124, in __init__
self.update_mappings()
File "C:\Instrument\Apps\EPICS\ISIS\inst_servers\master\DatabaseServer\moxa_data.py", line 138, in update_mappings
self._moxa_data_source.insert_mappings(*self._get_mappings())
File "C:\Instrument\Apps\EPICS\ISIS\inst_servers\master\DatabaseServer\moxa_data.py", line 104, in insert_mappings
self.mysql_abstraction_layer.update(
File "C:\Instrument\Apps\Python3\Lib\site-packages\genie_python\mysql_abstraction_layer.py", line 94, in update
self._execute_command(command, False, bound_variables)
File "C:\Instrument\Apps\Python3\Lib\site-packages\genie_python\mysql_abstraction_layer.py", line 199, in _execute_command
raise DatabaseError(str(err))
genie_python.mysql_abstraction_layer.DatabaseError: 1062 (23000): Duplicate entry 'MOXA_SELAB2.isis.cclrc.ac.uk-1' for key 'port_mappings.PRIMARY'
```

when it finds two moxas with different IPs but the same hostname in the registry. I don't think it should still work, it just needs to catch the error and make it more clear what's actually going on.

### acceptance criteria
- [ ] if i have two moxas in the registry with the same hostname but a different IP, the database server whinges but makes it clear what is actually wrong.