[DO] Unexpect output in sky check

[Michaelvll]

sky check shows a long output when DO dependencies are not setup. We should avoid the long error message, and make it more user friendly, e.g. see other clouds below

 DO: disabled                              
    Reason: Traceback (most recent call last):
  File "/workspaces/skypilot/sky/clouds/do.py", line 263, in check_credentials
    do_utils.client().droplets.list()
  File "/workspaces/skypilot/sky/provision/do/utils.py", line 94, in client
    _client = _init_client()
  File "/workspaces/skypilot/sky/provision/do/utils.py", line 56, in _init_client
    raise DigitalOceanError(
sky.provision.do.utils.DigitalOceanError: no credentials file found from the following paths ['/home/vscode/Library/Application Support/doctl/config.yaml', '/home/vscode/.config/doctl/config.yaml']

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/workspaces/skypilot/sky/adaptors/common.py", line 31, in load_module
    self._module = importlib.import_module(self._module_name)
  File "/opt/conda/envs/sky/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1004, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'pydo'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/workspaces/skypilot/sky/check.py", line 41, in check_one_cloud
    ok, reason = cloud.check_credentials()
  File "/workspaces/skypilot/sky/clouds/do.py", line 264, in check_credentials
    except do.exceptions().HttpResponseError as err:
  File "/workspaces/skypilot/sky/adaptors/common.py", line 64, in wrapper
    m.load_module()
  File "/workspaces/skypilot/sky/adaptors/common.py", line 36, in load_module
    raise ImportError(self._import_error_message) from e
ImportError: Failed to import dependencies for DO. Try pip install "skypilot[do]"

  OCI: disabled                              
    Reason: Missing credential file at ~/.oci/config. For more details, refer to: https://docs.skypilot.co/en/latest/getting-started/installation.html#oracle-cloud-infrastructure-oci
  Paperspace: disabled                              
    Reason: Failed to access Paperspace Cloud with credentials.
    To configure credentials, follow the instructions at: https://docs.skypilot.co/en/latest/getting-started/installation.html#paperspace
    Generate API key and create a json at `~/.paperspace/config.json` with 
         {"apiKey": "[YOUR API KEY]"}
    Reason: Credentials not found

Version & Commit info:

• sky -v: PLEASE_FILL_IN
• sky -c: PLEASE_FILL_IN

[Michaelvll]

cc'ing @asaiacai @cblmemo