entrypoint.py: Explicitly import exit from sys

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Why are the changes needed?

entrypoint.py calls exit(1) when the env var "FLYTE_FAIL_ON_ERROR" is set.
When using the Spark plugin for Databricks jobs,
this results in NameError: name 'exit' is not defined exception,
where a successful exit with return code 1 was expected.

Behavior confirmed for both Databricks Runtime 13.3 LTS (Apache Spark 3.4.1, python 3.10)
and Databricks Runtime 10.4 LTS (Apache Spark 3.2.1, python 3.8)

What changes were proposed in this pull request?

Explicitly importing the exit function from the sys module should fix this.

How was this patch tested?

Setup process

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• I updated the documentation accordingly.
• All new and existing tests passed.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.06%. Comparing base (7144ae9) to head (fa17fc0).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2259      +/-   ##
==========================================
- Coverage   83.78%   83.06%   -0.73%     
==========================================
  Files         332      287      -45     
  Lines       25093    22972    -2121     
  Branches     3690     3478     -212     
==========================================
- Hits        21025    19082    -1943     
+ Misses       3448     3268     -180     
- Partials      620      622       +2     

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