Provide error handling when configuring a bad base_url

[quba42]

Summary

If I configure a hostname without a schema (https://) for the base_url of the CLI, I get a stacktrace and difficult to understand error instead of a clear error message.

Steps to reproduce

1. On a clean install of the CLI run pulp config create --base-url "host.example.com" (Using https://host.example.com would be correct).
2. Run some CLI commands, e.g.: pulp rpm remote list

Expected behavior

A clear and concise error message, something like:

Error: Invalid server URL, please check the provided base_url.

Stacktrace/Error log/Actual behavior

# pulp rpm remote list
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/pulpcore/cli/common/openapi.py", line 82, in load_api
    with open(apidoc_cache, "rb") as f:
FileNotFoundError: [Errno 2] No such file or directory: '/root/.cache/squeezer/or.deploy7.dev.atix/api.json'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/bin/pulp", line 11, in <module>
    load_entry_point('pulp-cli==0.11.0', 'console_scripts', 'pulp')()
  File "/usr/lib/python3.6/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/usr/lib/python3.6/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python3.6/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python3.6/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/lib/python3.6/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/click/decorators.py", line 73, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/usr/lib/python3.6/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/click/decorators.py", line 73, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/usr/lib/python3.6/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/pulpcore/cli/common/generic.py", line 493, in callback
    result = entity_ctx.list(limit=limit, offset=offset, parameters=parameters)
  File "/usr/lib/python3.6/site-packages/pulpcore/cli/common/context.py", line 364, in list
    result: Dict[str, Any] = self.pulp_ctx.call(self.LIST_ID, parameters=payload)
  File "/usr/lib/python3.6/site-packages/pulpcore/cli/common/context.py", line 139, in call
    result = self.api.call(operation_id, *args, **kwargs)
  File "/usr/lib/python3.6/site-packages/pulpcore/cli/common/context.py", line 99, in api
    self._api = OpenAPI(**self._api_kwargs)
  File "/usr/lib/python3.6/site-packages/pulpcore/cli/common/openapi.py", line 68, in __init__
    self.load_api(refresh_cache=refresh_cache)
  File "/usr/lib/python3.6/site-packages/pulpcore/cli/common/openapi.py", line 87, in load_api
    data = self._download_api()
  File "/usr/lib/python3.6/site-packages/pulpcore/cli/common/openapi.py", line 109, in _download_api
    r: requests.Response = self._session.get(urljoin(self.base_url, self.doc_path))
  File "/usr/lib/python3.6/site-packages/requests/sessions.py", line 555, in get
    return self.request('GET', url, **kwargs)
  File "/usr/lib/python3.6/site-packages/requests/sessions.py", line 528, in request
    prep = self.prepare_request(req)
  File "/usr/lib/python3.6/site-packages/requests/sessions.py", line 466, in prepare_request
    hooks=merge_hooks(request.hooks, self.hooks),
  File "/usr/lib/python3.6/site-packages/requests/models.py", line 316, in prepare
    self.prepare_url(url, params)
  File "/usr/lib/python3.6/site-packages/requests/models.py", line 390, in prepare_url
    raise MissingSchema(error)
requests.exceptions.MissingSchema: Invalid URL '/pulp/api/v3/docs/api.json': No schema supplied. Perhaps you meant http:///pulp/api/v3/docs/api.json?

Pulp and pulp-cli version info

Pulp CLI version 0.11.0
pulpcore version 3.14.9

Additonal context

I was trying to get Pulp CLI to work on a Katello instance. Not sure if that is relevant.