[serve] Better validation for arguments to client.start()

[edoakes]

What is the problem?

If I pass in a bad host to serve, it gives me an obscure error:

(base) ray/python % serve start --http-host 0.0.0.
File descriptor limit 256 is too low for production servers and may result in connection errors. At least 8192 is recommended. --- Fix with 'ulimit -n 8192'
2020-12-10 18:50:14,006 INFO worker.py:653 -- Connecting to existing Ray cluster at address: 192.168.0.124:6379
(pid=10674) 2020-12-10 18:50:14,534     INFO controller.py:346 -- Starting HTTP proxy with name 'SERVE_CONTROLLER_ACTOR:SERVE_PROXY_ACTOR-node:192.168.0.124-0' on node 'node:192.168.0.124-0' listening on '0.0.0.:8000'
(pid=10677) Task exception was never retrieved
(pid=10677) future: <Task finished name='Task-5' coro=<HTTPProxyActor.run() done, defined at /Users/eoakes/code/ray/python/ray/serve/http_proxy.py:159> exception=gaierror(8, 'nodename nor servname provided, or not known')>
(pid=10677) Traceback (most recent call last):
(pid=10677)   File "/Users/eoakes/code/ray/python/ray/serve/http_proxy.py", line 167, in run
(pid=10677)     sock.bind((self.host, self.port))
(pid=10677) socket.gaierror: [Errno 8] nodename nor servname provided, or not known

Likely to be the same with a bad port number.

Reproduction (REQUIRED)

Please provide a short code snippet (less than 50 lines if possible) that can be copy-pasted to reproduce the issue. The snippet should have no external library dependencies (i.e., use fake or mock data / environments):

If the code snippet cannot be run by itself, the issue will be closed with "needs-repro-script".

• I have verified my script runs in a clean environment and reproduces the issue.
• I have verified the issue also occurs with the latest wheels.