Unhandled ProtocolError for remote host closing

[Sjoerd Kerkstra]

Follow up to #20

New server shows new ways in which connections can be forcefully and apologetically closed:

 File "C:\ProgramData\Miniconda3\envs\anonserver_p37\lib\site-packages\urllib3\response.py", line 458, in _error_ca
tcher
    raise ProtocolError("Connection broken: %r" % e, e)
urllib3.exceptions.ProtocolError: ("Connection broken: ConnectionResetError(10054, 'An existing connection was forci
bly closed by the remote host', None, 10054, None)", ConnectionResetError(10054, 'An existing connection was forcibl
y closed by the remote host', None, 10054, None))`

This error is not caught be the fix to #20 because it is not a requests error but a urllib3 error.
I'd like to not burden dicomtrolley-using software with handling this error seperately. Re-raise as TrolleyError