Add a check for S3 paths being incorrectly passed as an entry point

[laurenyu]

to address the concern in #471

Merge Checklist

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[codecov-io]

Codecov Report

Merging #500 into master will decrease coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #500      +/-   ##
==========================================
- Coverage   94.29%   94.16%   -0.13%     
==========================================
  Files          59       59              
  Lines        4610     4612       +2     
==========================================
- Hits         4347     4343       -4     
- Misses        263      269       +6

Impacted Files	Coverage Δ
src/sagemaker/estimator.py	90.57% <100%> (+0.06%)	⬆️
src/sagemaker/local/image.py	88.08% <0%> (-1.89%)	⬇️

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