Improve behaviour when the Eth node is unreachable

[ulope]

Problem Definition

If the Eth node is unreachable we currently throw a pretty gnarly exception (see below).
Instead we need to provide a helpful message to the user and exit gracefully.

Traceback (most recent call last):
  File "site-packages/urllib3/connection.py", line 171, in _new_conn
  File "site-packages/urllib3/util/connection.py", line 79, in create_connection
  File "site-packages/urllib3/util/connection.py", line 69, in create_connection
  File "site-packages/gevent/_socket3.py", line 336, in connect
ConnectionRefusedError: [Errno 111] Connection refused

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "site-packages/urllib3/connectionpool.py", line 600, in urlopen
  File "site-packages/urllib3/connectionpool.py", line 354, in _make_request
  File "http/client.py", line 1239, in request
  File "http/client.py", line 1285, in _send_request
  File "http/client.py", line 1234, in endheaders
  File "http/client.py", line 1026, in _send_output
  File "http/client.py", line 964, in send
  File "site-packages/urllib3/connection.py", line 196, in connect
  File "site-packages/urllib3/connection.py", line 180, in _new_conn
urllib3.exceptions.NewConnectionError: <urllib3.connection.HTTPConnection object at 0x7f70bd0c8f60>: Failed to establish a new connection: [Errno 111] Connection refused

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "site-packages/requests/adapters.py", line 445, in send
  File "site-packages/urllib3/connectionpool.py", line 638, in urlopen
  File "site-packages/urllib3/util/retry.py", line 398, in increment
urllib3.exceptions.MaxRetryError: HTTPConnectionPool(host='127.0.0.1', port=8545): Max retries exceeded with url: / (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f70bd0c8f60>: Failed to establish a new connection: [Errno 111] Connection refused',))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "raiden-script.py", line 2, in <module>
  File "site-packages/raiden/__main__.py", line 11, in main
  File "site-packages/click/core.py", line 722, in __call__
  File "site-packages/click/core.py", line 697, in main
  File "site-packages/click/core.py", line 1043, in invoke
  File "site-packages/click/core.py", line 895, in invoke
  File "site-packages/click/core.py", line 535, in invoke
  File "site-packages/click/decorators.py", line 17, in new_func
  File "site-packages/raiden/ui/cli.py", line 991, in run
  File "site-packages/raiden/ui/cli.py", line 891, in _run_app
  File "site-packages/click/core.py", line 535, in invoke
  File "site-packages/raiden/ui/cli.py", line 596, in app
  File "site-packages/raiden/network/rpc/client.py", line 229, in __init__
  File "site-packages/web3/version.py", line 14, in node
  File "site-packages/web3/manager.py", line 107, in request_blocking
  File "site-packages/web3/manager.py", line 90, in _make_request
  File "site-packages/web3/middleware/gas_price_strategy.py", line 18, in middleware
  File "cytoolz/functoolz.pyx", line 232, in cytoolz.functoolz.curry.__call__
  File "site-packages/web3/middleware/formatting.py", line 50, in apply_formatters
  File "site-packages/web3/middleware/attrdict.py", line 18, in middleware
  File "cytoolz/functoolz.pyx", line 232, in cytoolz.functoolz.curry.__call__
  File "site-packages/web3/middleware/formatting.py", line 50, in apply_formatters
  File "site-packages/web3/middleware/normalize_errors.py", line 9, in middleware
  File "cytoolz/functoolz.pyx", line 232, in cytoolz.functoolz.curry.__call__
  File "site-packages/web3/middleware/formatting.py", line 50, in apply_formatters
  File "cytoolz/functoolz.pyx", line 232, in cytoolz.functoolz.curry.__call__
  File "site-packages/web3/middleware/formatting.py", line 50, in apply_formatters
  File "cytoolz/functoolz.pyx", line 232, in cytoolz.functoolz.curry.__call__
  File "site-packages/web3/middleware/formatting.py", line 50, in apply_formatters
  File "site-packages/web3/middleware/exception_retry_request.py", line 87, in middleware
  File "site-packages/web3/providers/rpc.py", line 68, in make_request
  File "site-packages/web3/utils/request.py", line 26, in make_post_request
  File "site-packages/requests/sessions.py", line 559, in post
  File "site-packages/requests/sessions.py", line 512, in request
  File "site-packages/requests/sessions.py", line 622, in send
  File "site-packages/requests/adapters.py", line 513, in send
requests.exceptions.ConnectionError: HTTPConnectionPool(host='127.0.0.1', port=8545): Max retries exceeded with url: / (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f70bd0c8f60>: Failed to establish a new connection: [Errno 111] Connection refused',))
[21787] Failed to execute script raiden-script

[LefterisJP]

I'll quickly do this as I just got the same exception due to using a non-standard port and it 's indeed really ugly.