Don't raise exceptions when offline

[buhtz]

When I am offline pydoctor is of course not able to retrieve intersphinx files.

Currently this does raise an exception. This should not happen. Just print an error.

ERROR   -  Could not retrieve intersphinx object.inv from https://python-docx.readthedocs.io/en/stable/objects.inv
Traceback (most recent call last):
  File "C:\python\Lib\site-packages\urllib3\connection.py", line 174, in _new_conn
    conn = connection.create_connection(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\python\Lib\site-packages\urllib3\util\connection.py", line 72, in create_connection
    for res in socket.getaddrinfo(host, port, family, socket.SOCK_STREAM):
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\python\Lib\socket.py", line 962, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
socket.gaierror: [Errno 11001] getaddrinfo failed

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\python\Lib\site-packages\urllib3\connectionpool.py", line 714, in urlopen
    httplib_response = self._make_request(
                       ^^^^^^^^^^^^^^^^^^^
  File "C:\python\Lib\site-packages\urllib3\connectionpool.py", line 403, in _make_request
    self._validate_conn(conn)
  File "C:\python\Lib\site-packages\urllib3\connectionpool.py", line 1053, in _validate_conn
    conn.connect()
  File "C:\python\Lib\site-packages\urllib3\connection.py", line 363, in connect
    self.sock = conn = self._new_conn()
                       ^^^^^^^^^^^^^^^^
  File "C:\python\Lib\site-packages\urllib3\connection.py", line 186, in _new_conn
    raise NewConnectionError(
urllib3.exceptions.NewConnectionError: <urllib3.connection.HTTPSConnection object at 0x0000024AC15F0510>: Failed to establish a new connection: [Errno 11001] getaddrinfo failed

[tristanlatr]

Agreed, exit code should be 2 though.