gaierror(-2) is raised in the failure case of getaddrinfo. This is
compatible with cpython's socket module.
Typical session:
```
>>> import socketpool
>>> import wifi
>>> socket = socketpool.SocketPool(wifi.radio)
>>> try: socket.getaddrinfo("boo", 0)
... except socket.gaierror as e: ee = e
...
>>> type(ee)
<class 'gaierror'>
>>> ee.errno == socket.EAI_NONAME
True
>>> ee.strerror
'Name or service not known'
>>> raise ee
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 1, in <module>
gaierror: (-2, 'Name or service not known')
```
Closes: #6941
aef55b7e41