[nit] Friendlier error message when user hasn't installed ray[serve]

[architkulkarni]

If you run pip install ray in a fresh environment and run from ray import serve, you get the following error message:

>>> from ray import serve
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/archit/anaconda3/envs/serve-cuj/lib/python3.7/site-packages/ray/serve/__init__.py", line 1, in <module>
    from ray.serve.api import (start, get_replica_context, shutdown, ingress,
  File "/Users/archit/anaconda3/envs/serve-cuj/lib/python3.7/site-packages/ray/serve/api.py", line 14, in <module>
    from fastapi import APIRouter, FastAPI
ModuleNotFoundError: No module named 'fastapi'

We should detect when ray[serve] is not installed and raise an error message telling the user to run pip install "ray[serve]". Otherwise the user might end up responding to the current error messages by installing the missing packages (fastapi, etc) one at a time.