Better error message when core is not found (because its not even running)

[matthiasdebernardini]

I got this error message when I think the problem was core not running.

Traceback (most recent call last):
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 501, in _get_bitcoind_cookie_authpair
    with open(cookie_file, "r") as fd:
FileNotFoundError: [Errno 2] No such file or directory: '/Users/matthiasdebernardini/Library/Application Support/Bitcoin/testnet3/.cookie'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 3130, in <module>
    main()
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 3121, in main
    cli.run()
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 318, in run
    return fnc(*func_args, **func_kwargs)
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 2072, in cli_main
    ui()
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 282, in wrapper
    return fnc(*args, **kwargs)
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 2058, in ui
    config, walls = _get_config(require_wallets=False)
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 3090, in _get_config
    (conf, wallet_confs) = GlobalConfig.from_ini(conf_path, confp)
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 2476, in from_ini
    rpc = c.rpc(net_name=net_name)
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 2440, in rpc
    return get_rpc(
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 2597, in get_rpc
    got = _get_rpc_inner(url, **kwargs)
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 2619, in _get_rpc_inner
    return BitcoinRPC(
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 631, in __init__
    super(RawProxy, self).__init__(
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 441, in __init__
    authpair = self._get_bitcoind_cookie_authpair(
  File "/Users/matthiasdebernardini/gits/coldcore/./coldcore", line 510, in _get_bitcoind_cookie_authpair
    raise ValueError(
ValueError: Cookie file unusable ([Errno 2] No such file or directory: '/Users/matthiasdebernardini/Library/Application Support/Bitcoin/testnet3/.cookie') and rpcpassword not specified in the configuration file: '/Users/matthiasdebernardini/Library/Application Support/Bitcoin/bitcoin.conf'