Commands run without configuration fail with ugly stacktrace

[pasky]

Example:

pasky@gundi:~/elisctl$ elisctl csv get
Traceback (most recent call last):
  File "/usr/local/bin/elisctl", line 11, in <module>
    load_entry_point('elis-tools', 'console_scripts', 'elisctl')()
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/Users/pasky/elisctl/elisctl/csv.py", line 47, in download_command
    rsp = api_client.get(f"byperiod/{ceil(step.total_seconds())}/{int(start.timestamp())}")
  File "/Users/pasky/elisctl/elisctl/lib/api_client.py", line 82, in get
    return self._request_url("get", f"{self.url}/{path}", query)
  File "/Users/pasky/elisctl/elisctl/lib/api_client.py", line 58, in url
    _url = get_credential("url").rstrip("/")
  File "/Users/pasky/elisctl/elisctl/configure.py", line 40, in get_credential
    res = config["default"].get(attr)
  File "/usr/local/Cellar/python/3.7.2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/configparser.py", line 958, in __getitem__
    raise KeyError(key)
KeyError: 'default'

We should guide the user to configure elisctl first.