Added notification message and exception for starting a non-configured endpoint

[yongyanrao]

For Issue #432
After the fix, when we try to start a non-configured endpoint test-ep:

$ funcx-endpoint start test-ep

Endpoint test-ep is not configured!
1. Please create a configuration template with:
	funcx-endpoint configure test-ep
2. Update the configuration
3. Start the endpoint

[codecov]

Codecov Report

Merging #435 (ba12731) into main (2bb8175) will decrease coverage by 2.65%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #435      +/-   ##
==========================================
- Coverage   35.94%   33.29%   -2.66%     
==========================================
  Files          20       20              
  Lines        2245     2607     +362     
==========================================
+ Hits          807      868      +61     
- Misses       1438     1739     +301     

Impacted Files	Coverage Δ
...dpoint/funcx_endpoint/endpoint/endpoint_manager.py	56.62% <ø> (+1.15%)	⬆️
...cx_endpoint/funcx_endpoint/endpoint/interchange.py	28.66% <0.00%> (-3.52%)	⬇️
...ncx_endpoint/executors/high_throughput/executor.py	30.02% <0.00%> (-1.89%)	⬇️
..._endpoint/executors/high_throughput/interchange.py	11.48% <0.00%> (+0.17%)	⬆️

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[ZhuozhaoLi]

I think it should probably keep the old style. No need to raise an exception here, and this exception seems irrelevant.

[yongyanrao]

Tried to raise a similar exception as

FileNotFoundError: [Errno 2] No such file or directory: '/home/zhuozhao/.funcx/test_local_1/config.py'

We can keep the old style.

[yadudoc]

This looks good. Let's merge this.