starting non-existing endpoint does not tell people to first configure it

[ZhuozhaoLi]

It should check existence first and tell users to funcx-endpoint configure ep

(funcx) [zhuozhao@midway2-login1 ~]$ funcx-endpoint start test_local_1
Traceback (most recent call last):
  File "/home/zhuozhao/.conda/envs/funcx/bin/funcx-endpoint", line 10, in <module>
    sys.exit(cli_run())
  File "/home/zhuozhao/.conda/envs/funcx/lib/python3.6/site-packages/funcx_endpoint/endpoint/endpoint.py", line 169, in cli_run
    app()
  File "/home/zhuozhao/.conda/envs/funcx/lib/python3.6/site-packages/typer/main.py", line 214, in __call__
    return get_command(self)(*args, **kwargs)
  File "/home/zhuozhao/.conda/envs/funcx/lib/python3.6/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/home/zhuozhao/.conda/envs/funcx/lib/python3.6/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/home/zhuozhao/.conda/envs/funcx/lib/python3.6/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/zhuozhao/.conda/envs/funcx/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/zhuozhao/.conda/envs/funcx/lib/python3.6/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/home/zhuozhao/.conda/envs/funcx/lib/python3.6/site-packages/typer/main.py", line 497, in wrapper
    return callback(**use_params)  # type: ignore
  File "/home/zhuozhao/.conda/envs/funcx/lib/python3.6/site-packages/funcx_endpoint/endpoint/endpoint.py", line 94, in start_endpoint
    os.path.join(endpoint_dir, manager.funcx_config_file_name)).load_module()
  File "<frozen importlib._bootstrap_external>", line 399, in _check_name_wrapper
  File "<frozen importlib._bootstrap_external>", line 823, in load_module
  File "<frozen importlib._bootstrap_external>", line 682, in load_module
  File "<frozen importlib._bootstrap>", line 265, in _load_module_shim
  File "<frozen importlib._bootstrap>", line 684, in _load
  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 674, in exec_module
  File "<frozen importlib._bootstrap_external>", line 780, in get_code
  File "<frozen importlib._bootstrap_external>", line 832, in get_data
FileNotFoundError: [Errno 2] No such file or directory: '/home/zhuozhao/.funcx/test_local_1/config.py'