Connection reset by peer in mavproxy_map

I was using MAVProxy in ardupilot simulator ( `Ardupilot/ardupilot/Tools/autotest/sim_vehicle.py` ) and faced connection error when using mavproxy_map.

* My command:

```
ardupilot/Tools/autotest/sim_vehicle.py -j 2 -v ArduCopter -f X --console --map
```

* stdout:

```
Loaded module console
Loaded module map
Unable to find parameter 'FRAME'
Log Directory:
Telemetry log: mav.tlog
MAV> Waiting for heartbeat from tcp:127.0.0.1:5760
Fontconfig warning: ignoring UTF-8: not a valid region tag
STABILIZE> Received 696 parameters
Saved 696 parameters to mav.parm
Exception in thread Thread-2:
Traceback (most recent call last):
File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
self.run()
File "/usr/lib/python2.7/threading.py", line 754, in run
self.__target(*self.__args, **self.__kwargs)
File "/usr/local/lib/python2.7/dist-packages/MAVProxy/modules/mavproxy_map/mp_tile.py", line 287, in downloader
img = resp.read()
File "/usr/lib/python2.7/socket.py", line 355, in read
data = self._sock.recv(rbufsize)
File "/usr/lib/python2.7/httplib.py", line 612, in read
s = self.fp.read(amt)
File "/usr/lib/python2.7/socket.py", line 384, in read
data = self._sock.recv(left)
error: [Errno 104] Connection reset by peer
```

I think it caused by a rack of error handling when UDP connection failed.
Then I added try-except all errors when reading response, it worked fine.

* `MAVProxy/modules/mavproxy_map/mp_tile.py` diff:

```diff
@@ -284,7 +284,11 @@ class MPTile:
print("non-image response %s" % url)
continue
else:
- img = resp.read()
+ try:
+ img = resp.read()
+ except Exception as e:
+ print(e.message)
+ img = ""
```

Can I make this PR? Any concern about this?

Start in MAVProxy/modules/mavproxy_map/mp_tile.py, especially MPTile.downloader and the resp.read() call; reproduce with the supplied sim_vehicle.py command. Done means a reset during tile retrieval no longer terminates the downloader thread and the map module continues running.

Written by the indexing model from the issue text.