[UX] Add network failure handling for Lambda sky check

[romilbhardwaj]

If your internet is down (e.g., wifi is off) or lambda cloud is inaccessible, sky check would fail for lambda cloud with a fatal error:

 Checking Lambda...socket.gaierror: [Errno 8] nodename nor servname provided, or not known

During handling of the above exception, another exception occurred:

urllib3.exceptions.NewConnectionError: <urllib3.connection.HTTPSConnection object at 0x16d9c7e80>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known

During handling of the above exception, another exception occurred:

urllib3.exceptions.MaxRetryError: HTTPSConnectionPool(host='cloud.lambdalabs.com', port=443): Max retries exceeded with url: /api/v1/instances (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x16d9c7e80>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known'))

During handling of the above exception, another exception occurred:

requests.exceptions.ConnectionError: HTTPSConnectionPool(host='cloud.lambdalabs.com', port=443): Max retries exceeded with url: /api/v1/instances (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x16d9c7e80>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known'))
(base) ➜  sky-experiments git:(master) ✗ 

After:

  Lambda: disabled          
    Reason: Failed to verify Lambda Cloud credentials. Check your network connection and try again.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• sky check without wifi
• sky check with wifi